
Glass. 
Book. 



COPYRIGHT DEPOSIT 



REFRACTION 

AND 

MOTILITY OF THE EYE 

WITH CHAPTERS ON COLOR BLINDNESS 
AND THE FIELD OF VISION 

DESIGNED FOR STUDENTS AND PRACTITIONERS 

BY 

ELLICE M. ALGER, M.D., F.A.O.S. 

Professor of Ophthalmology at the New York Post- 
graduate Medical School, etc. 



WITH ONE HUNDRED AND TWENTY-FIVE ILLUSTRATIONS 



SECOND REVISED EDITION 




PHILADELPHIA 

F. A. DAVIS COMPANY, Publishers 

English Depot 

Stanley Phillips, London 

1920 






COPYRIGHT, 1910 
COPYRIGHT, 1920 

BY 
F. A. DAVIS COMPANY 

Copyright, Great Britain. All Rights Reserved. 



am i3 mo 



©CI.A576035 



PRESS OF 

F. A. DAVIS COMPANY 

PHILADELPHIA, U.S.A. 



PREFACE TO SECOND EDITION. 



This little book has been reprinted a number of 
times without change since its first appearance, but the 
recent remarkable recrudescence of interest in Ophthal- 
mology has made another edition desirable, with some 
changes and additions. 

Prepared originally as a series of lectures to the 
writer's post-graduate students it included a number of 
topics not usually found in books on refraction. Every 
effort was made to avoid the theoretical without sacrificing 
the essential; to have each subject prepare for and lead 
up to the following one; to avoid the laying down of dog- 
matic rules, but to explain reasons so that the student 
should eventually rely on himself and be prepared to 
undertake the entire functional examination of the eye. 

The common belief that Eefraction is nearer an exact 
science than any other department of medicine has been 
a great misfortune to Ophthalmology. On this postulate 
men with equal training should arrive at exactly the same 
results. It leads many ophthalmologists to consider re- 
fraction as professional drudgery, as uninteresting as the 
measurement of the schematic eye, except for the fee in- 
volved; and is the chief basis for the lay belief that it is 
not essentially a department of medicine at all. 

Refraction is more than a science. It is an art based 
on a science, and the pre-eminent scientist may notori- 
ously be a very poor artist. 

(in) 



j v PREFACE TO SECOND EDITION. 

Theoretically there are few eyes that could not be 
"glassed." But if the physician believes that glasses are 
a nuisance which, like drugs, should only be prescribed 
when clearly indicated; if he considers not the eyes alone 
but the whole patient and finally prescribes what he thinks 
that patient needs, whether it be simply ocular hygiene, or 
glasses, or an operation; if he then instead of washing 
his hands of the result like a mere scientist, watches it 
sympathetically and hopefully, he converts what might be 
drudgery into service and earns some of the proverbial 
joy of the artist. 

E. M. A. 



INTEODUCTIOE". 



The preparation of this work is the result of several 
years of post-graduate teaching, and while portions of it 
may be of interest to those who are already competent 
ophthalmologists, it is especially designed to meet what the 
writer conceives to be the needs of tAvo individuals, the 
general practitioner and the embryo ophthalmologist. 

Whatever may be our personal estimate of the per- 
manent value of writings such as those of Gould, Stevens, 
Eanney and others, we must all admit that they have 
worked a great change in the popular idea, whether lay or 
professional, of the scope of ophthalmology. The physician 
has always been ready to admit the value of fundus 
examinations in occasional cases of nephritis, meningitis 
and the like, but these occasions were so rare that he has 
thought best to devote his attention to diagnostic methods of 
wider application. He has always been ready to admit the 
necessity of glasses to those who could not see but, as he had 
neither the time nor the skill to provide them, he neglected 
the subject entirely. But of recent years a great change 
has taken place. A few men have long taught, almost 
unheeded by the profession, that the relation between the 
eves and other organs is so intimate, that defects in one 
often result in functional defects in the others. The laity 
first viewed the idea with incredulity, later with increased 
favor. Very possibly they will let their enthusiasm carry 
them too far, and lead them to expect too much, but at pres- 
ent there is a public demand so large and so profitable, that 

(v) 



v i INTRODUCTION. 

it "has attracted the attention of irregulars of all sorts as 
the advertising pages of every magazine will show. 

The physician is being compelled to investigate that he 
may meet the inquiries of his own patients, and even if he 
has no time or desire to enter the special field himself, he 
must have an intelligent conception of the possible role of 
the eyes in a given case. 

The limits of the field are not yet clearly denned: 
probably they will expand in some directions and contract 
in others, but they overlap more or less nearly every depart- 
ment of medicine. The pediatrist is beginning to under- 
stand that defective eyes are responsible for much of the 
mental hebetude of children, for truancy and dislike for the 
mental tasks which are normally attractive to the young, 
for headaches, habit spasms, and possibly for some 
epilepsies as well. The internist is coming to recognize 
that malfunction of the eyes may at times cause some of 
those vague conditions which it used to be the fashion to 
call vertigo, biliousness, and malaria and more recently 
uricacidemia, autotoxemia, and the like ; for many cases of 
functional neurosis of the stomach, the heart, the intestines. 
The neurologist must admit the eyes as a factor of impor- 
tance in man'y cases of headache, insomnia, and those per- 
sistent and discouraging states of mental and physical 
irritability or depression which he calls neurasthenia. The 
gynecologist is beginning to recognize that many of the 
headaches and nervous manifestations usually ascribed to 
the menopause are due rather to the simultaneous onset of 
presbyopia and are relievable by glasses. 

But we shall never know the exact limits of the field, 
except by a vast amount of more or less experimental 
collaboration between general practitioners and specialists, 
and if we expect to secure this collaboration we must give 



INTRODUCTION. vii 

in return results based on work that is both honest and 
skillful. In the first respect I do not doubt we can meet 
the demand but no one who makes a practice of investiga- 
ting the previous treatment of his new patients can avoid 
the conclusion that the general average of work is very far 
from what it should be. 

The reason for this is the almost universal lack of 
systematic instruction. The embryo specialist has too often 
with the lapse of years become very rusty in his physics, 
anatomy, and physiology. When he begins his post-grad- 
uate work in one of the large cities he is invariably im- 
patient of the preliminary drudgery which he ought to have 
mastered at home, and, fascinated by the variety of instru- 
ments and the facility with which others use them, he is apt 
to forget that the making of a good ophthalmologist re- 
quires much more than the possession of a complete office. 

The man who neglects the fundamentals in his eager- 
ness to work on patients, and to see and do operations, 
rarely acquires more than a smattering of his subject. 

By mastering the principles at home, one subject at a 
time, with a few simple instruments and a schematic eye, 
the student when he finally comes to his post-graduate work 
will be surprised at the ease with which he takes it up. 
He not only derives many times more benefit from his 
instructors but actually saves much time. 

There are two great classes of patients who consult the 
physician regarding their eyes : the first need glasses simply 
to improve their sight, while the second come to be relieved 
of headaches or reflex troubles of some kind. Though it 
requires a larger outfit, the task is in the first class often 
absurdly simple, and since the practitioner will certainly 
have more skill than the itinerant optician or department 
store clerk to whom most of these people now resort, he may 



v iii INTRODUCTION. 

without hesitation attempt their correction. The relief of 
the second class often demands an exactness of correction 
which is apparently beyond the reach of many professed 
ophthalmologists, and certainly should not be attempted by 
the general practitioner if other help is available. Even if 
he makes no attempt to correct personally the errors he dis- 
covers, he can often throw light on a puzzling case and per- 
haps bring new hope to some miserable sufferer. 

In conclusion the writer wishes to acknowledge his 
obligation to text-books such as those of G-anot, Foster, 
Fuchs and Ball, and to the special works of many others. 
It may seem an impossible task to harmonize the views of 
such dh^erse authorities as Gould, Stevens, Savage, Duane 
and Valk, but the writer can at least acknowledge his deep 
obligation to every one of them. From the last-named in 
particular he has derived through the personal contact of 
many years not only instruction and advice but the stim- 
ulating influence of an inquiring mind and a perennial 
enthusiasm. 

E. M. A. 



CONTENTS. 



Introduction 

CHAPTER I. 



PAGE 

iii 



Optics 



Light: Emission and wave theories. Reflection: Regu- 
lar and irregular — Its laws — Mirrors, plane, concave and convex. 
Refraction: Its laws — Prisms, their action, strength and 
numbering — Lenses composed of infinite number of prisms — 
varieties — Convex spherical lens — action — axial ray — focus — no- 
dal points — optic centre— secondary axes and foci — conjugate 
foci— images and their construction— real and virtual images- 
Concave spherical lenses — action, foci and images — Cylindrical 
lenses, convex and concave, action, axis, strength — Numbering 
of lenses— Metric and inch systems— Recognition — Centreing and 
measurement of lenses — Combinations. 

CHAPTER II. 

The Emmetropic Eye 37 

Anatomy: Supporting, nutritional and sensory structures. 
Refracting Media: Cornea, aqueous, lens and vitreous — 
Function of iris — The emmetropic eye — The standard or reduced 
eye — Formation of images — Optic axis — Visual angle — Distant 
vision — Near vision — Diffusion circles. Accommodation: Mech- 
anism and effect — Far point and near point — Region and 
amplitude of accommodation — Effects of age. Examination of 
the Patient: Personal history— Eye history, general scope — 
Test cards for distant and near vision— Records — The trial 
case. 

CHAPTER III. 

Ophthalmoscopy 62 

The Ametropic Eye: Varieties— Examination— Lateral il- 
lumination— Purkinje-Sanson reflexes. The Ophthalmoscope: 
Construction and theory — Direct method — Opacities in the re- 
fracting media, their localization— The pupil and its reaction — 
The fundus oculi — The nerve head and its physiological and 
pathological variations — Retinal and chorioidal vessels— The 
macula— Estimation of refraction with the ophthalmoscope — Em- 



(ix) 



x CONTENTS. 

PAGE 

metropia — Hyperopia — Myopia— Astigmatism — Differences cf level 
in the fundus. Indirect Method: Its technique — Advantages 
and disadvantages. 

CHAPTER IV. 

Retinoscopy . 88 

Retinoscopy: Its value as a test and advantages- 
Requisites: relaxed accommodation in patient and keen vision 
in surgeon — The mirror — The light — The reflex or shadow — Its 
real motion — Its apparent motion — High hyperopia — Myopia — Em- 
metropia — Myopia of one dioptre — Rule for retinoscopy — Estima- 
tion of astigmatism of various types — Difficulties — Corneal and 
lenticular opacities — Spherical aberration — The scissors reflex. 
Retinoscopy Without a Cycloplegic not an exact but a 
useful test Rule. 

CHAPTER V. 

The Pupil — Cycloplegics — Miotics — Static Refraction ... Ill 

The Pupil: Its contraction and dilation — Various reactions 
— Anisocoria — The Argyll-Robertson pupil. Cycloplegics and 
Mydriatics and their action — Indications for cycloplegia in 
refraction — Choice of a cycloplegic — Atropin and homatropin 
Static Refraction — Its meaning and best method of estimat- 
ing — Use of the trial case — Final prescription as based on static 
refraction. Miotics: Their action and use — Ciliary tonics. 

CHAPTER VI. 
Hyperopia . . 127 

Hyperopia: Definition — Refractive and axial — Vision in hy- 
peropia — Manifest — Latent — Total — Facultative — Absolute hyper- 
opia — Near vision — Symptoms, subjective and objective — Diag- 
nosis—Treatment — Retinoscopy — Ophthalmoscopy — Keratometry 
— Trial case — Illustrative cases — Hyperopia simulating myopia- 
Complicating strabismus — Spasm of accommodation — When to 
use cycloplegics and when to avoid them — Final prescription 
for glasses — General rules. 

CHAPTER VII. 
Myopia 144 

Myopia: Definition — Refractive and axial— Causes of my- 
opia — The myopic a diseased eye— Anatomical changes in 
chorioid and papilla — The myopic crescent — Symptoms of 
myopia— Region and amplitude as compared to those of emine- 



CONTENTS. xi 

PAGE 

tropia an<i hyperopia— Advantages and disadvantages of myopia 
—Postponement of presbyopia— Progressive myopia— Lack of 
reflex symptoms and pain in pure myopia — Prognosis — Ciliary 
spasm and myopia — Diagnosis of myopia— Differentiation of be- 
nign from progressive myopia by the corneal radius — Treat- 
ment of myopia — Danger of over-correction — Advantages of cy- 
cloplegia — Retinoscopy— Ophthalmoscope unreliable in myopia — 
Subjective tests— Full or partial correction — Hygiene — Operative 
method in high myopia — Probable reduction of myopia by re- 
moval of lens. 

CHAPTER VIII. 
Astigmatism 163 

Irregular Astigmatism: Corneal and lenticular types 
—Their limited treatment. Regular Astigmatism: Corneal 
and lenticular— Refraction — Causes — Vision in various types — 
Asthenopia and other symptoms— Estimation by retinoscopy, 
the ophthalmometer and trial case — Cycloplegia or not — The 
ophthalmometer or keratometer — Its advantages and disad- 
vantages — Astigmatic charts. Htperopic Astigmatism: Sim- 
ple and compound — Detection and correction both with and with- 
out cycloplegia. Myopic Astigmatism: simple and compound 
and their correction. Mixed Astigmatism. 



CHAPTER IX. 

Presbyopia — Anisometropia — Aphakia 194 

Presbyopia: What it means and what it depends on — 
Symptoms, subjective and objective — Treatment in emmetropia — 
In ametropia of various types. Anisometropia: Symptoms 
subjective and objective— When to treat and when to let alone. 
Aphakia: Congenital and operative — The fitting of glasses 
after cataract extraction. 

CHAPTER X. 

Binoci lab Vision 208 

Binocular Single Vision: Its physiology— The eyeball 
and its possible movements— Primary position — Convergence — 
Divergence — Rotation— Torsion— The extrinsic muscles and their 
action singly and together— Their nerve supply — Basal centres 
— Fusion centres for co-ordination — Cortical centres for volun- 
tary rotation— Tests of binocular vision — Diplopia, homonymous, 
crossed and vertical. 



xii CONTENTS. 

CHAPTER XL FAGE 

Normal Motility 219 

Position of Rest at far and near points — Prism tests — 
Maddox rod — Duane's parallax test — Stevens', lens and Hardy's 
cone tests — Torsion equilibrium— Maddox rods and Maddox 
double prism — Relation of convergence and accommodation — 
Metre angle — Cover test — Graefe 'equilibrium test. Fusion 
Powers: Requisites for fusion — Prism convergence — Di- 
vergence — Sursumvergence — Deorsumvergence — Torsion power 
— Stevens' clinoscope — Maddox rods— Relation of fusion powers. 
Voluntary Motions: Rotations — Linear measurement— Use 
of perimeter and Stevens' tropometer. 

CHAPTER XII. 
Heterophoria 242 

Orthophoria: Meaning and rarity. Heterophoria: 
Varieties and nomenclature — Causes, congenital and acquired 
— Symptoms, subjective and objective — Treatment— Refraction — 
Increase of fusion power or nerve stimulus — Increase of muscle 
power — The stereoscope and prism exercises — Prisms for con- 
stant use — Operation a last resort — Partial tenotomy — Complete 
tenotomy — Shortening an ocular muscle — Advancement— Indica- 
tions for and choice of operation. Esophoria: Its varieties, 
diagnosis and treatment. Exophoria — Hyperphoria — Cyclo- 
phoria and the various conjugate imbalances. 

CHAPTER XIII. 

Heterotropia — Strabismus — Squint 270 

Strabismus: Primary and secondary deviations — Diagnosis 
from paralytic squint — Measurement, — linear, — by perimeter — 
Amblyopia in squint, congenital and acquired — Apparent squint 
— Varieties of squint— Etiology, congenital and acquired— Re- 
fraction an important factor — Lack of fusion power — Organic 
changes in muscles — Treatment of patients who are not ambly- 
opic — Refraction— Education of fusion powers — Worth's ambly- 
oscope and the stereoscope — Operation the last resort— Choice of 
operation — Results, happy and unhappy — Test of cure — Treat- 
ment of amblyopic cases — Cosmetic effect only — Refraction of 
good eye— Operation on bad eye— Choice of operation. 

CHAPTER XIV. 
Ocular Paralysis 296 

Motor Paralyses: Primary and secondary deviations — 
Diplopia— False orientation— Limited motion — Vertigo — Contrac- 



CONTENTS. xiij 

l'AGE 

tures — Varieties of paralysis — Etiolcgic factors — Localization of 
lesion — Localization by diplopia — Theoretical effect of various 
single paralyses— Practical method— Means of excluding or 
identifying paralysis of obliques — Rule for identifying straight 
muscles involved — The. perimeter and tropometer in paralyses — 
complicated cases— Causal treatment— Local treatment by pads 
and prisnis — Operation in old paralyses. Fusion Paralyses: 
Of convergence and divergence — Symptoms — Diagnosis — Treat- 
ment. Conjugate Paralyses to right and left and up and 
down — Symptoms — Diagnosis — Treatment. 



CHAPTER XV. 

Color-Blixdness 319 

Colors: Spectral and pigmentary — Primary and secondary. 
Color-Sexsatioxs: Young, Helmholz and Hering theories. 
Color-Blixdxess: Congenital and acquired — Significance of 
latter varieties — Tests — Spectroscope — Lantern tests — Wool tests k 
— Rules — Quantitative testing. 

CHAPTER XVI. 
Field of Vision 333 

Central and peripheral vision — Projection and orientation- 
Confrontation test — Application in cataract — Perimeter test — 
Perimeter charts — Fields for white and the color fields — Sco- 
tomata of various types— Diagnostic value of color fields- 
Characteristic fields in glaucoma — Atrophy — Retinitis pig- 
mentosa — Retrobulbar neuritis — Chorioiditis — Neurasthenia — 
Hysteria— Binocular defects in brain lesions— Course of optic 
nerve— Effect of interruption at various points on field and 
pupil— Hemianopsia and its significance— Homonymous — Nasal 
—Temporal. 

CHAPTER XVII. 

The Relation of Functional Eye Diseases to General 
Medicine 354 

EYE-STRAIN: Its definition— Objective symptoms— Subjec- 
tive symptoms— Ocular headaches— Migraine— Epilepsy— Chorei- 
form movements— Neurasthenia— Functional disturbances of 
metabolism— Complicating the menopause— Spinal resultants- 
Spasmodic torticollis — Differential diagnosis — Treatment. 



x i v CONTENTS. 



CHAPTER XVIII. 

PAGE 

Ocular Malingering 367 

Purpose of tests — General principles — Objective tests — 
Subjective tests — Pupillary indications — Fusion tests — Diplopia 
tests — Exclusion tests — Color tests — Stereoscopic tests — Field 
of vision. 

Index 379 



Refeaotion" and Motility 
or the Eye. 



CHAPTER I. 
OPTICS. 

Light may be defined as that form of energy which, by 
its stimulation of the retina, excites the sensation of vision. 
Every luminous body emits light. The older corpuscular 
or emission theory of light taught that infinitesimal par- 
ticles were given off by such bodies which traveled with 
infinite velocity. The more modern . undulatory theory 
presupposes an agitation of the luminiferous ether which 
travels in all directions from its source in the form of 
oscillations or waves. 

On the emission theory the particles were supposed to 
be in actual motion, like bullets discharged from a gun, 
while on the undulatory theory there is no actual motion 
of the particles themselves, but only a state of disturbance 
which manifests itself by a series of waves with transverse 
vibrations like those caused by striking a stretched wire. 
In the illustration (Fig. 1) the straight lines show the 
direction in which the rays of light travel from a luminous 
body, while the curved lines show the manner of progress. 
These waves may have different heights and lengths, and 
produce varying sensations in proportion to these, but 
always travel in straight lines, so long as the medium is of 
a constant demit v. 

(i) 



2 REFRACTION AND MOTILITY OF THE EYE. 

A ray of light may be defined as the smallest conceiv- 
able portion of light. Eays of light starting from their 
source and proceeding in all directions must necessarily be 
more or less divergent, but it is evident that rays coming 
in the same general direction from a source infinitely dis- 
tant will diverge so little that they may be considered as 
parallel, and for our purposes rays from a source twenty 
feet distant may be considered practically parallel. A 
pencil of light consists of a number of rays practically 




Fig. L 



parallel to a central ray, while a learn of light is simply a 
larger collection of parallel rays. Though rays of light 
always travel in straight lines while in a homogeneous 
medium, they cease to do so when the medium changes, and 
undergo reflection, refraction, absorption and dispersion, 
the amount of each depending on the nature of the new 
medium. 

When rays of light are absorbed, they cease for the time 
to exist as light, but are transformed into heat, or may be 
given off again in the form of fluorescence or of phosphores- 
cence. Absorption of light is generally associated with 
reflection. Appreciation of color is due to the absorption 



OPTICS. 3 

by the object viewed of all the light except those rays which, 
when reflected or transmitted, excite in the retina the 
sensation of some definite color. Thus, a red glass absorbs 
all the light except the red rays. 

The dispersion of light does not concern us especially. 
It is sufficient to say that it is by the scattered rays of light 
that non-luminous objects are visible, and it is due to the 
reflection of rays from very irregular surfaces. 

Reflection is inseparable from refraction. If an object 
were so absolutely transparent that all rays of light passed 
through it, it would be invisible. Plate glass is perhaps 
the nearest approach to perfect transparency, and yet many 
rays are reflected from the most perfect of plates. 

When a ray of light impinges on a surface, the angle 
which it makes with a perpendicular to this surface is the 
angle of incidence, while the angle it makes in passing away 
is the angle of reflection. 

The angles of incidence and reflection are always equal, 
and the incident and reflected rays are always in a plane 
perpendicular to the reflecting surface. The two laws may 
be demonstrated by the apparatus represented in Fig. 2. 
It consists of a graduated circle in a vertical plane. Two 
brass slides move around the circumference, on one of 
which is a ground glass screen, P, and on the other an 
opaque screen, N, with a small central opening. Fixed to 
the latter is a small mirror, which can be adjusted, but 
is always in a plane perpendicular to the plane of the 
graduated circle. At the centre of the circle a small plane 
mirror is placed horizontally, 0. 

A pencil of sunlight is reflected by the mirror 
through the aperture N so as to fall upon the mirror 0, 
whence it is reflected and caught on the screen P, which is 
moved to the necessary position. The number of degrees 



4 



REFRACTION AND MOTILITY OF THE EYE. 



in the arc AN will be found equal to those in AP. 
Therefore, the angle of incidence, AON, must be equal 
the angle of reflection, A OP, and since the plane of the 
circle is perpendicular to the plane of the mirror by 
construction, the plane of the rays must also be perpen- 
dicular to it. This can be proven to be true for any other 
position of N, which causes a corresponding change in the 



Jy/$ 


I 


9/ 




/^ 




«//yi\>'i' - t > f 




J \ 








\ 


\ 


















R 




^' 





Fig. 2. 



position of P. If the source of light, N, is moved to A 
perpendicular to 0, the screen, P, would have to be moved 
to the same spot to intercept the reflection. Consequently, 
when light is reflected back to its source, its path must, be 
exactly perpendicular to the reflecting surface at the point 
of reflection. 

Fig. 3 shows two rays of light from a common source 
reflected from points C and D of a plane surface. The 
angles aCA and a'CA, being the angles of incidence and 



OPTICS. 



reflection of one ray, must be equal. Similarly aDB and 
a"DB must be equal. Then the angles aCA and aDB 
of incidence, must have the same relation to each other as 
the angles of reflection which are their exact equivalents. 
Therefore, rays from a common source after reflection from 
a plane surface are equally divergent after reflection. By 




C D 

Fig." 3. 

supposing that rays from a' and a" are convergent, we can 
show that they are equally convergent after reflection, and 
the case is easier still if we suppose parallel rays, since 
the angles would all be equal. 

Let MN represent a plane mirror on which rays of 
light from the object, AB, are reflected to the eye. Eays 




Fig. 4. 

from A appear to come from a, while rays from B appear to 
come from b. Therefore, the object, AB, appears to be 
situated at ab, behind the mirror, and it can easily be 
proven to be as far behind MN as the object actually is in 
front of it. Such an image is called a virtual one. 

Since the distance of objects from the eye .is estimated 
with the aid of experience by the angle which rays coming 



6 REFRACTION AND MOTILITY OF THE EYE. 

from them to the eye make with each other, which is 
unchanged by plane mirrors (see "Visual Angle"), the 
apparent distance of A B from the eye is equal to the sum of 
the incident and reflected rays. The case is very different, 
however, if the reflecting surface is a rough one. If we 
substitute in Fig. 4 a lamp for the object and a printed 
page for the mirror the rays, striking the page undergo the 
irregular reflection or dispersion alluded to before. We 
get no image of the lamp but the page becomes itself a 
source of light from which rays emanate and enter the eye 
as though they came from MN. The page remains visible 
so long as light falls upon it no. matter what change is made 
in the position of the lamp. On this principle we illuminate 
the fundus of the eye by a mirror and its details become 
visible, and since the illuminated area acts as a source of 
light we can estimate the refraction by tracing the course of 
the emergent rays, disregarding entirely the course of the 
entering ones. 

The appearance of images in a mirror of objects which 
we are accustomed to consider as having a right and left 
side is changed. For instance, in the reflection of a person 
in a plane mirror the right side of the body is reflected in 
the right side of the mirror, but as the image appears to face 
the observer, a motion of the right hand will appear in the 
mirror like a corresponding motion of the left hand on the 
part of the reflection. The same thing occurs in the reflec- 
tion of letters on a page or test card. This is known as 
lateral inversion. 

Reflections from concave mirrors are based on exactly 
the same laws of the equality of the angles of incidence and 
reflection (Fig. 5). 

If MN represents such a concave mirror, with parallel 
rays of light impinging on it, and C the centre of curvature, 



OPTICS. 7 

the ray, CF, passing through the centre of curvature must 
be perpendicular to MN and will be reflected back from F, 
the vertex or middle of the mirror. CF is then the 
principal axis of the mirror, while any other rays passing 
through C must also be perpendicular to the mirror at the 
point of intersection and constitute secondary axes. Eays 
parallel to the principal axis after reflection will intersect 
the principal axis at , the angles of incidence and reflec- 
tion being equal. The other parallel rays will intersect the 
principal axis at the same point which is known as the 
focus of the mirror, and its distance from F as the focal 




Fig. 5. 

length. The focal length of weak concave mirrors is 
approximately half the radius of the curvature. 

Eays of light from infinity are parallel and after 
reflection will all pass through the focus, and, conversely, 
rays emanating from its focus will after reflection emerge 
parallel. As the source of rays approaches the mirror, the 
rays become more divergent and come to a focus nearer and 
nearer to the* centre of curvature; when the rays emanate 
from the centre of curvature, being perpendicular, they are 
all reflected back to their source. When the light is 
between C and 0, the rays cut the principal axis further 
and further from the mirror, till it reaches 0, when they 
become parallel, and as the light is carried still nearer the 
mirror, the rays become divergent and their focus a virtual 
one formed behind the mirror by projection of the rays. 



8 REFRACTION AND MOTILITY OF THE EYE. 

Hitherto it has been supposed that the luminous object 
placed in front of a mirror was simply a point, but if the 
object is larger we can imagine it as composed of a number 
of points, each on a secondary axis, and by locating the focus 
of each we should determine the position of the image of 
the object. 




Fig. 6. 

The reflection of AB in a concave mirror is constructed 
thus. A ray, AD, parallel to the principal axis of the 
mirror, is reflected so as to pass through the principal focus, 
and the point where it meets the secondary axis, drawn 
from A, through the centre of curvature, (7, will be the 
point where the image of A is formed, namely, a. In a 
similar way we locate the image of B at b, then all the 
imaginary points between A and B will be reflected so as 
to fall between a and b. 

This image may be seen in two ways, either by placing 
the eye at ab, or it may be intercepted on a screen at the 
same place; therefore the image is real, inverted, smaller 
and placed between the centre of curvature and the prin- 
cipal focus. If we place the object at ab between the centre 
of curvature and the focus, the image will be at AB, larger, 
inverted and real. If the object is placed at the focus, no 
image is formed, since the rays after reflection will be 
parallel to the principal axis. 

If the object is at AB (Fig. 7) within the focus, the 
reflection of A will occur at D and that of B at E. Conse- 



OPTICS. 



9 



quently, the only place where the secondary axes, CA and 
CB, can intersect these lines is behind the mirror at a and h, 
which represent the extremities of the image. This image 
is then virtual in that it cannot be projected on a screen, it 
is larger than the object and is erect. 




Fig. 7. 

If the mirror be a convex one, the rays are divergent, 
no matter how near or how far AB is from the mirror. 
Consequently, the only points where these rays could inter- 
sect the secondary axes, AC and BC, is at the two points 
behind the mirror, which determine the position of the 



^£- 




Fig. 8. 



image. Consequently, the reflection in a convex mirror is 
always erect, small and virtual. 

Refraction. — When rays of light pass from one 
medium to another of different density, they are refracted 
or bent. 

A very good illustration of refraction is the apparent 
bending of a cane thrust obliquely into a pool of water. 



10 



REFRACTION AND MOTILITY OF THE EYE. 



Let A, B and C represent three silver coins, so 
arranged on the bottom of an empty pail, that the light 
from the candle reflected from B comes to the eye of the 
observer and makes it visible. The other coins are invisible, 
because one (A) receives no light from the candle and the 
other (C) is hidden behind the edge of the pail. But if 
the pail be filled with water all three coins at once become 
visible. If the sides of the pail be considered as per- 
pendicular, we see at once that the rays from the candle 
passing from the air into a denser medium have been 




Fig. 9. 



refracted or bent toward the perpendicular so as to 
illuminate the coin, A. On the other side of the pail we 
see that the rays reflected from the coin, 0, when they pass 
from the the water into a medium of lesser density, have 
become bent from the perpendicular so as to reach the eye 
of the observer. We shall find this true in the case of 
other media than air and water, and the greater the differ- 
ence in their density, the greater the refraction, and the 
law of refraction can be stated thus. When a ray of light 
passes from one medium to another of different density in 
a direction perpendicular to the surfaces, it is not refracted; 
when its course is not perpendicular, it is bent toward the 



OPTICS. 11 

perpendicular in the denser medium and away from it in 
the lighter medium. The amount of refraction between 
two media is always the same and in the same plane, as 
can be demonstrated by a modification of the apparatus 
used in showing the law of reflection. (See Fig. 2.) 

The plane mirror in the centre of the graduated circle 
is replaced by a semi-cylindrical glass vessel of water at 
the exact centre of the circle. The pencil of light from N 
is refracted on passing into the water at 0, but passes out 
without refraction, because its direction is perpendicular to 
the curved bottom of the glass vessel. The screen, P , is 
moved along the arc till it intercepts the pencil at P'. 

A line, J, at right angles to AO, meeting the incident 
ray is known as the sine of the angle of incidence AON, 
while a similar line, B, is the sine of the angle of refraction, 
BOP'. The length of these sines can be read off on two 
graduated rules movable so as to be always horizontal, and 
while these lengths will vary with the size of the angles, they 
always maintain an exact proportion. For instance, a ray 
passing from air into water, the sine of the angle of 
incidence will be to the sine of the angle of refraction, as 4 
is to 3, while if the ray were traveling in the reverse 
direction, the proportion would be 3 to 4. This is known 
as the index of refraction and varies between different 
media. That of air to glass is 3/2 or 1.5. 

When a luminous ray passes from one medium into 
another of less density, as from water into air, the angle of 
incidence is always less than the angle of refraction. It 
therefore follows that there must be one angle of incidence 
of such value that the emergent ray would be exactly 
parallel to the surface at OB and rays, as from P, making a 
greater angle would not emerge at all, but be reflected at the 
surface toward Q (Fig. 10). This angle beyond which total 



12 



REFRACTION AND MOTILITY OF THE EYE. 



reflection occurs is known as the critical angle, and as there 
is no loss of light from absorption or transmission, the 
reflection is the most brilliant possible, and therefore this 




Fig. 10. 

method is frequently used in optical instruments. The 
critical angle from water to air is 48° 35', and that from 
glass to air 41° 48'. 

When a ray of light passing through the air impinges 
on a medium with greater density but with two surfaces, 
the same rule holds good. 




Let ABCD represent a section of plate glass with 
parallel sides. A ray of light striking perpendicularly will 



OPTICS. 13 

pass through without refraction, while if it strikes, obliquely 
it is bent toward the perpendicular when it enters the glass, 
and again from the perpendicular when it leaves, and since 
the surfaces of the glass are parallel, the ray passes on in a 
course exactly parallel to its former one. When the glass is 
a thin one, the amount of displacement is so slight that it is 
commonly ignored. 

If, however, the sides of the glass are not parallel, but 
approach each other, we have what is called a Prism, the 
angle of the sides being known as the angle of the prism. 

Let A B and BC represent the sides of a prism of glass, 




Fig. 12. 

AC being the base of the prism and B the apex. The angle 
ABC is known as the refracting angle of the prism. A 
ray of light from a candle, D, impinging on the side AB, is 
refracted toward the perpendicular, and when it passes 
through the side BC into a lighter medium is again 
refracted, this time away from the perpendicular. If the 
ray enters perpendicular to either surface of the prism, the 
refraction all takes place at the other and is the greatest 
possible. If the ray passes in such a direction that its 
course is parallel to the base, the angles of incidence and 
emergence are equal and the total refraction is the least 
possible. The light from the candle D is bent, so as to 
come to the observer at F and appears to him to come from 
E. A prism therefore bends rays of light toward its base 



14 



REFRACTION AND MOTILITY OF THE EYE. 



and causes an apparent displacement of the object toward 
the apex. 

The refracting power of a prism is directly in propor- 
tion to the refracting angle of the prism. The total 
deviation is measured by the angle formed by the direction 
of the incident and emergent rays, and in prisms of ten 
degrees is equal to half the angle of the prism, but in 
stronger prisms the angle of deviation increases. 

Prisms whose principal section is an isosceles right 
angled triangle afford a good example of total reflection. 



Fig. 13. 

In such a prism, ABC, a ray of light, 0, which enters per- 
pendicular without refraction and makes with the face, AB, 
an angle equal to B, or 45°. But, since the critical angle 
for glass is 41°45', it cannot emerge, but undergoes total 
reflection and emerges in the direction I, perpendicular to 
AC. 

In order that any ray refracted by the first face of a 
prism may emerge from the second, the refractive angle 
of the prism must be less than twice the critical angle of 
the prism substance. In glass, therefore, whose critical 
angle is 41°48', objects cannot be seen through a prism 
greater than 82° 96', or less than a right angle. 

Prisms were formerly numbered in degrees according 
to the refracting angle made at the intersection of their 



OPTICS. 15 

surfaces, but, as two prisms with exactly the same refract- 
ing angle might be of dissimilar strength owing to a 
difference of density in the glass, two other methods have 
been proposed by which the number of the prism depends 
on its power of deflecting the rays of light. 

Method of Dennett. — If we take a circle of any 
diameter and mark off on its circumference a distance 
exactly equal to its radius of curvature, this will always 
represent an angle of 57.295 degrees and is called the arc 
of the radian. A prism base down at the centre of curva- 
ture which will deflect a ray of light downward exactly 
Vioo P ai "t of the arc of the radian is called a centrad, 
designated by a small triangle, base up V. The centrad 
then causes the deflection of rays of light .57295° or 
approximately half a degree. 



Fig. 14. 

The Prentice Method adopts as the standard the 
prism dioptre which causes the deviation of light 1 centi- 
metre for each metre of distance from the prism, the 
abbreviation being PD, or graphically A- This is a very 
convenient method for use in the consulting room. For 
instance, a prism of 5 A strength would deflect rays of 
light 5 cm. at one metre, 10 cm. at two metres and 30 cm. 
at six metres, which is the usual working distance. 

For practical purposes it makes no difference which 
scale is used in one's trial case, since the three systems are 
almost exactly equivalent up to 20°, while the centrad and 
prism dioptre, which are almost universally used, do not 
vary materially up to 35° or 40°. 



16 



REFRACTION AND MOTILITY OF THE EYE. 



Neutralizing Pbisms. — To find the strength of an 
unknown prism one should look through it at a line at any 
convenient distance, thus: 




Fig. 15. 

Since prisms cause an apparent displacement of 
objects toward their apices, the edge of the glass on the 
right must be the apex of the prism. If, then, we place 
over this prism others of known strength, with the base 
of one over the apex of the other till the line looked at is 
again continuous, the two prisms must be of exactly equal 
strength; or we can view through the prism a series of 
ABC 3) E V 





Fig. 16. 

parallel lines one centimetre apart and one metre distant. 
Each space of displacement of the first strong line indi- 
cates one prism dioptre. Such a "prismometre scale" can 
be calculated for any convenient distance. 

Lenses.— A lens is a portion of transparent substance 
having one or both surfaces curved. Lenses are commonly 



OPTICS. 17 

considered as being made of glass, but it is not necessarily 
so. A bead of perspiration, for instance, forms a diminu- 
tive, but powerful, lens for collecting the sun's rays. 

Spherical Lexses are so called because one or both 
surfaces are curved like sections of a sphere and may be 
either convex or concave. 

Every convex lens may be considered as being made up 
of an infinite number of prisms with their bases toward the 
centre of the lens. 

Consider in Fig. 17 such a lens in which each half has 
three of its component prisms indicated with an infinite 
number of imaginary prisms occupying the intervening 
intervals. 




Fig. 17. 

The ray of light from A striking perpendicularly on 
both surfaces of the lens passes through without refraction 
to A'. The rays from B and B f , parallel to A, impinge on 
the first prism of which the sides are almost parallel and 
are l>ent toward the bases of the prisms or the centre of the 
lens and are again bent slightly on passing out on the 
other side >o that both unite with the original ray at A'. 

The rays C and C impinge on prisms which are further 
from the centre arid consequently stronger, and the refrac- 
tion is greater and the rays join the others at A', 



18 REFRACTION AND MOTILITY OF THE EYE. 

In a convex spherical lens then a ray of light A A' 
which is perpendicular to both surfaces is called the axial 
ray and passes through without refraction, while rays 
parallel to the axial ray are bent more and more strongly 
toward the axial ray, as they are more and more distant 
from it, till they all come together at a point on the axial 
ray, called the focus of the lens. 

In very thick lenses the rays of light which pass 
through the periphery are refracted much more than the 
central ones so that they come to a focus inside the actual 
focus. This is known as spherical aberration, but in the 
thin lenses used in ophthalmology it is so slight as to be 
negligible. 

When an object is viewed through a convex spherical 
lens as the lens is moved in one direction the object 
appears to move in the opposite. The lens being made up 
of prisms with their bases toward the centre, all objects 
except those seen through the exact centre seem displaced 
toward the edge of the glass, and the prismatic action being 
progressively stronger, the displacement becomes greater 
and greater as the object is seen through the periphery. 
The stronger the lens, the more rapid is the "against" 
motion of an object viewed through it. 

If we suppose a convex lens of considerable thickness, 
the ray which strikes both surfaces perpendicularly passes 
through without refraction. This ray, AF, is the axial ray 
and its path the 'principal axis, and the point on the prin- 
cipal axis where the parallel rays come together is the 
principal focus F. Other rays which strike one surface 
obliquely if they enter and emerge from points whose 
tangents are parallel to each other, undergo exactly the 
same displacements as in passing obliquely through a 
window glass. They are refracted toward the perpendicular 



OPTICS. 



19 



on entering, and away from it on emerging, and pass on in 
a course exactly parallel to the first one. There are two 
points on the axial ray so situated that oblique rays directed 
toward one will appear to come from the other after lateral 




Fig. 18. 

displacement. These points, nn, are known as nodal points 
of the lens ; the paths of the rays directed toward the nodal 
points are known as the secondary axes, A'F' ; and the point 
where the secondary axes cut the principal axis is the 
optical centre of the lens, 0. Every lens has two nodal 
points and an optical centre, but in the thin lenses of 
ophthalmology the nodal points are practically coincident 




and constitute the optical centre, so that it is practically 
true that all rays that pass through the optical centre, aside 
from the principal axis, constitute secondary axes and are 
not refracted, while all rays parallel to a secondary axis 



20 REFRACTION AND MOTILITY OF THE EYE. 

come to a focus on it after emergence, such foci being 
called secondary foci (Fig. 19). 

The distance between the optic centre of a lens and its 
focus, is the focal length of the lens and is the chief 
means of indicating its strength. If a given lens brings 
parallel rays of light to a focus at a given point, a candle 
placed at that point, C, will send out rays which, passing 
through, emerge parallel. If the candle is brought nearer 
the lens, the rays must emerge divergent, while if the candle 
is carried beyond the focus of the lens, the rays will emerge 
convergent and come to a focus on the principal axis. 




a' >r 



Fig. 20. 

These two foci A and A', B and B' , are called conjugate foci 
from the fact that rays emerging from one will invariably 
come to a focus at the other, and as one conjugate focus 
approaches the lens on one side, the other recedes toward 
infinity. When one conjugate focus is just twice the focal 
length away from the optical centre, its fellow will be an 
exactly equal distance away oh the other side. 

This applies not only to the principal axis, but also to 
the secondary axis, each focus of which has its conjugate. 
This enables us to understand the construction of images. 

Images From Convex Lenses. — Let AB represent a 
candle at any convenient distance from the lens. Kays 
from the point A passing through the optical centre con- 
tinue without refraction, while other rays from the same 
point passing through other parts of the lens intersect the 
axial ray at A', which is the conjugate focus of A. In the 



OPTICS. 



21 



same way the conjugate focus of B is at B'. The infinite 
number of focal points on the candle between A and B have 
their corresponding conjugates between A' and B' and an 
image of the candle is formed there which is real in the 
sense that it will be visible if a screen be placed at that 




Fig. 21. 

point. It is also inverted and much smaller than the 
original. If the candle is approached toward the lens, the 
image formed on the other side recedes, at the same time 
growing larger, but it is still inverted and real. But when 
the candle is placed so that it is within the principal focus 
of the lens, the rays of light becoming more and more 
divergent form no focus. 




Fig. 22. 

If .47? represent the object, rays from A through the 
optic centre are unrefracted, but the other rays from A 
through the periphery of the lens are not refracted enough 
to intersect the axial ray AC and are divergent, the only 
meeting place being by their projection backward to A'. 
Rays from B are also divergent and only meet at B'. The 
image A'B' then is on the Game side of the lens as the object, 



£2 REFRACTION AND MOTILITY OF THE EYE. 

is larger than the object and is erect. Such an image which 
is formed by the prolongation backward of the divergent 
rays is called a virtual image and cannot be projected on a 
screen, but is visible to the eye when looking through the 
lens. 

The relative sizes of images and objects is in proportion 
to their distance from the optical centre of the lens. 

Concave Lenses may be considered as being made up 
of an infinite number of prisms with their bases outward. 
The ray of light, AFA', which passes perpendicularly 



r^v 



Fig. 23. 

through the optical centre of the lens, is the axial ray and 
is unrefracted, while all rays parallel to the axial ray are 
bent outward increasingly as they are nearer to the 
periphery of the lens. Since parallel rays are divergent 
after refraction through a concave lens, they do not meet to 
form a positive or real focus, but by being projected back- 
ward toward their source as in Fig. 23, they form an 
imaginary or negative or virtual focus, F. 

Conjugate foci are formed by concave lenses by projec- 
tion backward so that they are always virtual and on the 
same side of the lens. The secondary axes are those formed 
by rays passing through the optic centre without being per- 
pendicular or normal to the lens. They pass through as 



OPTICS. 23 

in convex lenses with a slight lateral displacement which 
is commonly disregarded, while rays parallel to the 
secondary axes emerge divergent. The conjugate foci on 
the secondary axes, as on the principal axis, are on the same 
side of the lens and are virtual. 

Images formed by concave lenses. — If AB represents 
an object at any given distance from a concave lens, C being 
the optic centre, all rays from A, except those passing 
through C, are refracted and emerge divergent, and, if pro- 
jected, form a virtual focus at A'. Rays from B undergo 




Fig. 24. 

a corresponding change and form a virtual conjugate focus 
at B' , while rays from the intermediate points between A 
and B form foci between A' and B'. Therefore the object 
AB forms an image at A'B' , which is a virtual image and 
cannot be projected on a screen and is always on the same 
side as the object, erect and smaller. Such an image can 
be seen by the eye as in the figure. It must be remembered 
that all the rays from AB, no matter what part of the lens 
they strike, form the virtual foci at A' and B' but that 
most of these are so divergent as to entirely miss the eye 
which is conscious of only the ones near the centre of the 
lens. Rays from A, which reach the eye after refraction, 



24 



REFRACTION AND MOTILITY OF THE EYE. 



appear to come from A', and those from B appear to come 
from B'. Therefore, the observer looking through a con- 
cave glass sees an image which is always erect and smaller 
than the object, and the further the object, the smaller the 
image. A concave glass is, then, always a minifying glass. 
Since a concave lens is made up of prisms with the base 
out, when the lens is moved from side to side, all objects 
seen through it are apparently displaced toward the centre 
of the lens, and, therefore, appear to move in the same 
direction as the lens, and the stronger the lens the greater 
the motion. 




Fig. 25. 



Cylindrical Lenses, usually called cylinders (abbre- 
viation, cyl.), are sections cut from a cylinder parallel with 
its axis, and are either convex or concave. 

The figure represents a convex cylinder about the axis, 
AB, which is vertical. A ray of light which strikes per- 
pendicularly so as to pass through the axis, A, is not 
refracted, but passes on through A'. All other rays which 
are parallel to A A' in the vertical plane also pass through 
without refraction and pass, still parallel, through the line 
A'B'. All the rays parallel to A A' in the horizontal 
meridian are refracted toward A A' and come to a focus at 
A'. Similarly all the parallel rays in the infinite number 



OPTICS. 25 

of horizontal planes between A and B are refracted toward 
their own axial ray and come to a focus at corresponding 
points on A'B f . Evidently then a convex cylinder refracts 
only rays of light in the meridian at right angles to its 
own axial plane, and its focus, instead of being a point as in 
a spherical lens, is a line parallel to its axis. If the rays 
come from a point of light at an infinite distance, the rays 
passing through the vertical plane would be parallel and the 
line A'B' would exactly equal in length AB, and the dis- 
tance from the optical centre of any horizontal section, say 
that of A and A' , would be the focal length of the cylinder; 
if the light approached, the rays of light would be 
divergent; those in the plane of the axis would emerge 
unchanged, while those at right angles to the axial plane 
would be less and less convergent. Consequently, the 
image line A'B' would tend to become longer and more 
distant, till it approached infinity, but still a real image. 

When the point of light is at a distance equal to the 
focal length of the lens, the rays in the axial plane would 
be very divergent, while those at right angles would emerge 
parallel after refraction; consequently no image would be 
formed. When the light is approached within the focal 
length of the cylinder, rays of light would be divergent in 
both planes and the focal line would be formed by projec- 
tion. The image then would be a line on the same side as 
the object, in other words a virtual instead of a real one. 

Any object looked at through a convex cylinder is 
unchanged in the diameter corresponding to the axis of the 
cylinder and magnified in the meridian at right angles. 
For instance, if a small circle be view r ed through a cylinder 
with the axis vertical, its vertical diameter appears 
unchanged, while its horizontal one is increased, making it 
appear oval. A convex cylinder may be said to consist of 



26 REFRACTION AND MOTILITY OF THE EYE. 

prisms with their bases toward the axial plane and increas- 
ing in strength toward the periphery. Consequently, if in 
viewing a fixed object through it, the cylinder be moved up 
and down in the line of its axis where there is no refraction, 
the object seems stationary, while if the cylinder be moved 
from side to side, the object is projected toward the apices 
of the prism and appears to move in the opposite direction 
to the movement of the cylinder, and the stronger the 
cylinder the more rapid the motion. 

Furthermore, if a line be viewed through the cylinder 

Fig. 26. 

held in such a position that the line coincides in direction 
with the axis of the cylinder, the portion of the line seen 
through the cylinder will appear continuous with that seen 
outside the cylinder, as will a line at right angles with the 
first line. 

But if the cylinder be rotated slightly wheel-fashion to 
the right, the upper portion of the vertical line above the 
centre appears displaced toward the thinner edge of the 
cylinder through which it is seen, while the lower half of 
the line appears displaced in the direction of the other edge, 
through which it is visible. 

When crossed lines at right angles are viewed as in 
the above figure, the horizontal line also appears displaced 



OPTICS. 27 

toward the thinnest part of the cylinder. In all these 
displacement tests with convex lenses, whether spherical or 
cylindrical, they should be held reasonably close to the eye, 
since otherwise, if they are strong and the object viewed is 
distant, the rays will cross before reaching the eye forming 
at their junction an aerial image which moves in the 
opposite direction. 

A Concave Cylinder is one which is thickest on the 
edges and thinnest at the centre, and may be considered as 
made up of prisms with their bases outward. 

Kays of light from a point of light at infinity, when 
they coincide with the axis of the cylinder, are not 
refracted, while those in all the planes at right angles to 
the axis emerge divergent. Their focus as in the concave 
spherical lens is found by projecting them back to their 
point of junction. The focus of the concave cylinder is a 
negative one, and no matter how close the light is brought 
to the lens, the rays emerge divergent, and the image 
formed is a virtual one. When seen through such a lens, an 
object appears to have its size unchanged in the diameter 
corresponding to the axis, while the diameter at right 
angles seems smaller. A circle viewed through a concave 
cylinder with the axis vertical appears oval, because its 
transverse diameter is lessened. When an object is viewed 
through a concave cylinder moved in the direction of its 
axis, the object remains stationary, but if the cylinder is 
moved from side to side at right angles to the axis, since it 
is composed of hypothetical prisms with the bases out, the 
object is displaced toward the centre and seems to move in 
the same direction in which the cylinder is moving, the 
rapidity indicating the strength of the cylinder. 

If two lines crossing each other at right angles are 
viewed through a concave cylinder held in such a way that 



28 REFRACTION AND MOTILITY OF THE EYE. 

one of the lines coincides with the axis of the cylinder, the 
position of both lines seen through the glass is continuous 
with the portions outside the glass. If now the cylinder 
be rotated slightly wheel-fashion to the right, the upper 
half of the line is displaced toward the axis of the glass and 
appears carried to the right, while the lower half is also 
displaced toward the axis and is carried to the left an equal 
amount. Consequently, the portion of line seen through 
the glass seems to rotate on its centre as a pivot in the 
same direction in which the cylinder is being rotated. For 
the same reason the line at right angles to the axis tends to 
keep its extremities in line with the thinnest edge of the 
glass and appears to rotate in the opposite direction, the 
amount of torsion being in direct proportion to the strength 
of the lens. 




The cylinders used in the trial case commonly have the 
axis indicated by a mark at each end and many have the 
edges ground parallel to the axis as shown in Fig. 26. The 



OPTICS. 29 

position of the cylinder before the eye is described by its 
relation to the horizontal and vertical planes of the head. 
For this purpose the cells in the trial frame are graduated 
in degree^ beginning at the (observer's) right of each cell. 
A cylinder whose axis was horizontal would be recorded as 
being at the axis, 0, or more commonly 180. A vertical 
axis would be axis 90, while all the intermediate positions 
possible can likewise be indicated in degrees. 

Less commonly trial frames are numbered from the 
vertical meridian which is marked and the degrees 
marked up to 90 on both sides. 

Cylinder axis vertical is equivalent to axis 90 on the 
other scale, and, if the axis is inclined inward or outward, 
it is indicated 15° nasal or temporal, as the case may be. 

Numbering of Lenses. — Lenses are numbered accord- 
ing to their focal lengths and it was formerly the custom 
to express this distance in inches. For instance, a convex 
lens which would bring parallel rays of light to a focus at 
1 inch from its optic centre was the standard of comparison 
and called an inch lens. A lens which had a focal length 
of two inches, being only half as strong as the standard, 
was numbered 54- A four-inch lens was expressed as 
having % the power of the unit and so on, the denominator 
of the fraction indicating the focal length of the lens in 
inches. The disadvantages of the system were first that the 
inch varies materially in different countries, and second, 
the inconvenience of expressing combinations most com- 
monly used, as for instance, + % + %2- The inch 
system is seldom employed nOw, being replaced by the Metric 
System. 

The dioptre which is the metric unit indicates 
a lens of such strength as to bring parallel rays of light 
to a focus at a metre (100 cm.), 39.37 inches, commonly 



30 REFRACTION AND MOTILITY OF THE EYE. 

considered 40 inches. A2 D. lens is just twice as strong 
and has a focal length of 50 centimetres, or 20 inches. A 
10 D. lens would be ten times as strong and would have a 
focal length of approximately 10 centimeters (4 inches), 
and so on. The fractions of a dioptre which it is often 
necessary to use, are expressed decimally. Thus a + .25 D. 
lens would be one quarter as strong as the 1 D. and 
would have the focal point 400 cm. or about 160 inches 
distant. 

To change from the inch system to the dioptric, divide 
the unit 40 by the focal length of the lens in inches. For 
instance a 20-inch lens would be equivalent to 40 -=-20 = 
2 D. A 13-inch would equal 40 -f- 13 = 3 D. almost. 

A concave lens of 1 dioptre (written — 1 D.) is a lens 
having a negative focal length of 1 metre. 

A convex cylinder of 1 D. strength brings parallel rays 
of light to a focus on a line 1 meter distant, while a con- 
cave cylinder of the same strength causes parallel rays of 
light to diverge as though they came from a line 1 meter 
behind the lens. 

To Find the Optical Centre oe Spherical Lenses. 
— The most convenient way is to draw on a sheet of paper 
two lines crossing each other at right angles and to view 
them through the lens (Fig. 26) . When the lens is in such 
a position that the portions of the crossed lines visible 
through the lens are continuous with the extraneous por- 
tions, the optical centre of the lens will be opposite the 
apparent intersection of the lines. When the cross is 
opposite any other point of the lens, one or both of the lines 
will appear displaced toward the thin part of the lens. 

Recognition and Measurement of Lenses. — If the 
crossed lines be viewed through a given lens which is moved 
from side to side and up and down, if the centre of the 



OPTICS. 31 

cross appears to move in the opposite direction to the 
motion of the lens in all directions, the lens is a convex one 
and its strength can be ascertained by placing over it con- 
cave glasses of known strength from the trial case till the 
two neutralize each other, when the cross will appear as 
through a plane glass — without motion. Thus, if a lens is 
exactly neutralized by a — 2D. in all meridians, it must 
have a strength of exactly + 2 D. If the cross appears to 
move in the same direction as the lens, it must be a concave 
one of the same strength as the plus lens which neutral- 
izes it. 

If the centre of the cross moves against the lens in one 
meridian and apparently has no motion in the other, the 
glass must be a simple convex cylinder, and if the lens be 
rotated wheel-fashion, till the lines seen through the lens 
are continuous with the parts outside, the axis of the cylin- 
der will correspond to the direction of the line which under- 
goes lateral displacement on moving the glass, and the 
— glass from the trial case which will exactly neutralize this 
displacement is the measure of the strength of the cylinder. 
Thus, if through a given lens the cross lines appear 
stationary when the lens is moved up and down, and to 
move against when the lens is moved from side to side, we 
rotate the lens till the vertical line appears continuous with 
the part outside the lens which will correspond with the 
axis, and disregarding the other meridian entirely since we 
know it to be plane, we find the lens of known strength 
which will stop the lateral motion of the line. If this be 
stopped by a — 2, the cylinder must have a strength of 
-(- 2 in this meridian. 

If the cross appears to move against the lens in both 
meridians, the lens is evidently convex in both. If rotation 
of the glass causes no torsion of the lines as in Fig. 26, the 



32 REFRACTION AND MOTILITY OF THE EYE. 

lens is a convex sphere and is readily neutralized. If tor- 
sion of the lines does take place, we must have a cylinder in 
combination with a sphere, in which case we determine the 
axis of the cylinder as before and neutralize that and the 
meridian at right angles separately. If the lateral motion 
of the vertical line is stopped by a — 3 and the up and 
down motion of the horizontal line by a — 2, the glass in 
question must have a strength of -j- 2 -f- 1 cyl. axis 90 or 
vertical. 

Concave sphero-cylindrical glasses may be measured in 
the same way except that the motion of the lines is with the 
lens and they are neutralized by + lenses. 

Occasionally glasses are met with in which the motion 
is against in one meridian and with in the other. The 
axes of the glass are to be found and each one neutralized in 
exactly the same way. Thus, if the vertical axis is 
neutralized by a — 3 and the horizontal by a + 2, we have 
a combination of a + 3 cylinder with a — 2 cylinder with 
axes at right angles to each other. 

Since the strength of a lens, the index of refraction 
being known, depends on the curvature of its surfaces, 
mechanical means of estimating this curve and the lens 
power have been devised (Fig. 28). In instruments of this 
type the two outside pins are immovable and are placed 
firmly on the lens, while the central one is pressed up in 
proportion to the convexity, or allowed to project by the 
concavity, the strength being automatically recorded on the 
dial as convex or concave. Both sides must be measured 
and added together to get the total strength of the lens. 
For instance : — 

and + 2 would indicate a planoconvex of + 2 D. ; + 
3 + 3 would indicate a double convex of + 6 D. ; — 2 + 5 
would indicate a convexconcave of + 3 D. 



OPTICS. 



33 



In combinations of spheres and cylinders one is always 
on one surface and the other on the reverse. In measuring 
the cylindrical side of the lens, rotate the lens measure on 
its central pin. as an axis, till the dial indicates the 
maximum amount when the axis will be at right angles to 




Fig. 28. 



the position of the pins, and if tried in the same meridian 
the dial will register 0. 

Combination of Lexses. — The sign of combination 
is 3- 

Combining Spheres. — Any number of spherical lenses 
placed with their surfaces touching and their optical centres 
over each other will be equivalent to a single lens of a 
strength equal to their sum. Thus, + 2 D. 3 + 3 D - . C 
+ 1 D. = + G I). ; or — 5 D. C — 2 D. = — 7 D. 

If a plus lens be placed over a minus of equal strength, 
the refraction will be nothing, since one will exactly 



34 REFRACTION AND MOTILITY OF THE EYE. 

neutralize the other. Thus, + 2 D. C — 2 D. = 0, but + 

4 D. 3 — 2 D. = + 2 D., since the minus glass only neu- 
tralizes half the stronger plus one. 

Combining Cylindrical Lenses. — Any number of 
cylindrical lenses placed together with their axes in the 
same meridian will be equal to a cylinder of the strength of 
their sum. Thus, + 3 cyl ax. 90 C + 2 cyl. ax. 90 = + 

5 cyl. ax. 90, or — 5 cyl. ax. 180 C — 2 cyl. ax. 180 = — 7 
cyl. ax. 180, or —4 cyl. ax. 180 + 2 cyl. ax. 180=— 2 
cyl. ax. 180. 

Two cylinders of the same strength and denomination 
with axes at right angles to each other are equal to a 
sphere of the same strength. Thus, + 2 ax. 90 C + 2 ax. 
180 = -f- 2 sph., since the + 2 ax. 90 will converge all the 
rays into a vertical line, while the + 2 ax. 180 will converge 
all the rays in the line to a central point. 

Conversely, any sphere may be considered as being 
made up of two cylinders of equal value with their axes at 
right angles. 

Two cylinders of different strength, but of the same 
denomination, with axes at right angles, will be equivalent 
to a sphere and a cylinder. For instance, +2 ax. 90 ^ 
+ 1 ax. 180 = + 1 sph. + 1 ax. 90. 

This can be shown by a convenient diagram (Fig. 29). 

The + 2 ax. 90 as shown above is equal to 2 lenses of 
+ 1 ax. 90 each. If we combine one of these with the + 1 
ax. 180, we shall have a sphere of + 1 D. and have the 
other -j- 1 ax. 90 left ; hence the combination may be 
written + 1 -f- 1 ax. 90. In the same way + 5 ax. 90 + 
3 ax. 180 = + 3 sph. C + 2 ax. 90, since + 5 ax. 90 is 
the equivalent of -f- 3 ax. 90 3 + 2 ax. 90 and the + 3 ax. 
90 C + 3 ax. 180 = + 3 sph. and we have + 2 ax. 90 left. 

The same sort of diagram may be used to estimate 



OPTICS. 



35 



other combinations of cylinders and sphero-cjdinders. For 
example, + 4 ax. 45 + 2 ax. 135. 

The + 4 ax. 45 is equivalent to two lenses of + 2 ax. 
45, one of which combines with the +2 ax. 135 to form a 
+ 2 sph. leaving still + 2 ax. 45. Therefore, the com- 
bination is equal to + 2 sph. C + 2 ax. 45. 

Example : + 2 sph. — 2 ax. 180 ? The + 2 sph. 
is the equivalent of + 2 ax. 90 + 2 ax. 180, the last half of 
which is neutralized by the — 2 ax. 180, leaving the 
simplest form of the combination as + 2 ax. 90. 




Fig. 29. 




Example: + 1 sph. C — 1 ax. 180? The + 1 sph. 
is equivalent to + 1 ax. 90 + 1 ax. 180. The + 1 ax. 180 
is neutralized by — 1 ax. 180 leaving + 1 ax. 90. 

Example: + 3 sph. C — 2 ax. 180 ? The + 3 sph. 
= + 3 ax. 90 Z. + 3 ax. 180, the last being partly 
neutralized by — 2 ax. 180, leaving + 3 ax. 90 3 + 1 ax. 
180, which is equal to + 1 sph. + 2 ax. 90. 

Crossed Cylinders are glasses having a plus cylinder 
at one and a minus cylinder at the opposite axis. They 
are seldom ordered, since it is very much easier both in the 
fitting and making of lenses to use sphero-cylinders which 
shall be exactly equivalent. 



36 REFRACTION AND MOTILITY OF THE EYE. 

Example: + 3 ax. 90 3 — 3 ax. 180? To secure 
the + 3 in the axis 90, we use a + 3 sph. and with a — 
cylinder at the ax. 180 so strong as not only to neutralize 
the sphere at that meridian, but to have a — 3 left. 
Evidently this can be accomplished by combining in this 
form + 3 sph. 3 — 6 ax. 180, or the same thing is 
accomplished by — -3 sph. 3 + 6 ax. 90. These two 
formulas are equivalent to each other, but the first is rather 
preferable as being lighter in weight. 

The Toric Lens is a meniscus (Fig. 16F), in which 
the concave side (a — 6 D.) is placed next the eye, while 
the anterior surface is made enough stronger or weaker to 
give the combination the required strength. Thus a — 6 
combined with a + 8 would be in effect a + 2 D. Cylin- 
drical corrections can be incorporated with either surface. 
Toric lenses can be measured by the same methods as flat 
lenses. Their chief advantage is that in whatever direction 
the eye is moved it looks nearly perpendicularly through the 
lens and so avoids a distortion that is unavoidable in flat 
lenses and which increases with their strength and with the 
obliquity of the gaze through them. For this reason they 
are sometimes called "Periscopic." 

"Puriktal" and "Katral" lenses are refinements of the 
same idea. 



CHAPTEE II. 

THE EMMETROPIC OR IDEAL EYE. 

Xo attempt will be made in a book of this character to 
deal exhaustively with the anatomy of the eye, with which 
the student is assumed to be already familiar. At the same 
time it may be well to review briefly the essential factors. 

The human eye consists of several tissues or groups of 
tissue, each of which has a distinct function to perform. 
Outside is the Sclera (Fig. 30), a tough, fibrous envelope 
which gives form to the eye and by affording a stable 
attachment to the muscles, makes the motion of the eyeball 
in various directions possible. The sclera has two open- 
ings : a large one anteriorly, into which the cornea is 
inserted, and a small one posteriorly through which the 
optic nerve enters the eye. 

Lining the sclera is the Chorioid (Fig. 30), which is 
for all essential purposes the nutritional layer of the eye. 
It is, therefore, composed chiefly of several layers of 
blood vessels, the larger ones being next the sclera and the 
smaller ones and capillaries next the retina and vitreous, 
which it is designed to nourish in part. The chorioid is 
also pierced by a posterior opening, corresponding to the 
entrance of the optic nerve, but does not extend quite as 
far forward as the sclera, terminating in an indentated 
anterior border called the ora serrata, where it is intimately 
associated with the ciliary body and iris. The chorioid also 
has an inner layer of dense brown pigment, which performs 
the function of absorbing light and preventing the internal 
reflection of ravs of light. 

(37) 



38 REFRACTION AND MOTILITY OF THE EYE. 

Running forward from the ora serrata toward the 
cornea is the ciliary body, triangular in its cross section, 
and terminating anteriorly just behind the cornea in about 
seventy ciliary processes, with depressions between them. 
The part of the ciliary body next the sclera is composed of 
the longitudinal and circular fibres of the ciliary muscle, 
while the ciliary processes lying on the muscle are very 
vascular and are the principal sources of the aqueous 
humor. 




Fig. 30. 

Lining the chorioid is the Retina, which may be con- 
sidered as the expansion of the optic nerve and is, there- 
fore, the essential organ of sight. In the living subject it 
is a thin, transparent membrane of a purplish red color. 
After death it rapidly becomes opaque, and as its visual 
purple bleaches out, it appears as a very delicate white 
membrane. It extends forward in all directions within 
the chorioid, as far as the ora serrata, and excepting at the 
head of the optic nerve and the ora serrata, simply 



THE EMMETROPIC OR IDEAL EYE. 39 

lies upon the chorioid without being attached to it. The 
retina does not really end at the ora serrata, but is con- 
tinued in a . simpler form over the ciliary body and the 
posterior surface of the iris. There are two points that are 
particularly noteworthy in the retina: one is the small 
white disk which lies somewhat to the inner side of the 




Fig. 31. 

posterior pole of the eye and marks the head of the optic 
nerve from which the retinal vessels emanate, and the 
second point of interest lies near the posterior pole of the 
eye. It is distinguished by its faint yellow color, whence 
the name macula lutea. At its centre is a slight depression 
which is the most sensitive portion of the retina and is 
called the fovea centralis. 



40 REFRACTION AND MOTILITY OF THE EYE. 

The retina will be considered as to some of its points 
of interest in other chapters. Images of objects are thrown 
upon it, and it is its function to change the vibrations of 
the luminiferous ether into nerve stimuli which shall be 
intelligible to the brain. Just how this takes place, we 
do not know, but it is certain that under the action of 
light the visual purple undergoes chemical changes by 
which it is decolorized. It is very probable that other 
chemical changes take place which have so far escaped 
discovery. 

If the eye consisted only of the tissues already spoken 
of, vision would exist, but it would consist only of the 
perception of light as a glow. Useful vision requires the 
focussing of light and the formation of images on the 
retina, which is only possible through the aid of refracting 
media which we shall now consider. 

The Cornea (see Fig. 30) is set into the anterior open- 
ing in the sclera, of which it may virtually be considered a 
transparent portion, since the microscope shows that the 
tissues of one pass over into the other imperceptibly. The 
cornea is perfectly transparent, except in disease and old 
age, is slightly elliptical in shape and somewhat thinner 
in the centre than at the periphery. Its radius of curva- 
ture is 7.5 mm. approximately, which is very much less 
than that of the sclera. Consequently, the cornea projects 
forward notably. In addition to the scleral tissue present 
in the cornea, there is an anterior portion representing the 
continuation of the conjunctiva, and a posterior which 
belongs to the ciliary body and iris. The cornea acts as a 
convex collecting lens by the aid of the aqueous. 

The Crystalline Lens (Fig. 30) lies within the circle 
formed by the ciliary processes, but in such a way that its 
border is distant half a millimeter from the apices of those 



THE EMMETROPIC OR IDEAL EYE. 41 

processes. It is suspended by a ligament, or zonula ciliqris, 
which together with the lens divides the eyeball into two 
cavities : a larger posterior containing the vitreous humor, 
and a smaller anterior containing aqueous. 

The lens is' transparent and colorless, and has some- 
what the appearance of a biconvex lens, the posterior sur- 
face of which is very much more curved than the anterior. 
The lens is enclosed in a transparent capsule and is com- 
posed of concentric layers of varying density, those at the 
centre having a regularly increasing density and refracting 
power. The optical function of the lens consists in bring- 
ing the rays of light that are already convergent from 
passing through the cornea still further together, till they 
focus on the retina. 

The Aqueous (Fig. 30) is a transparent, thin fluid, 
probably secreted by the iris and ciliary body which lies 
between the cornea and the lens. It has practically the 
same density as water, and its function is one of nutrition 
as well as refraction. A moment's consideration will show 
that the cornea has one convex surface and one concave, 
which would practically neutralize each other as do the 
sides of a watch crystal; but the presence of the aqueous 
serves to nullify the action of the posterior surface to a 
large degree, and the cornea and aqueous act together as a 
powerful convex lens. 

The Vitreous Humor (Fig. 30) fills the posterior part 
of the eye. It is a transparent, jelly-like substance, 
inclosed in the meshes of an equally transparent reticulum. 
The vitreous is surrounded by a structureless, transparent 
membrane, called the hyaloid membrane; it presents a 
fossa for contact with the posterior surface of the lens and 
is in contact with the retina. It has a still further con- 
vergent action on rays of light. 



42 REFRACTION AND MOTILITY OF THE EYE. 

The Iris. — The cornea and lens, with the vitreous and 
aqueous acting together, have an action on rays of light 
like that of a very powerful convex lens, and we have seen 
that in powerful lenses the peripheral rays of light are 
refracted so that they come to a focus sooner than the 
central rays, constituting the so-called spherical aberration 
which is a source of confusion, and that if these peripheral 
rays be excluded, the image is much more distinct. As in 
the camera or microscope, the image formed through a 
pin-hole aperture is just as distinct if the light is 
bright enough, and it is the function of the iris to 
regulate the amount of illumination and cut off the- 
peripheral rays. 

The iris is a disk-shaped membrane with a central 
opening, the pupil. It springs peripherally from the 
anterior portions of the ciliary body, from which point it 
stretches over the lens, its pupillary border resting on the 
anterior capsule upon which it slides during the movements 
of the pupil. The iris consists essentially of numerous 
blood vessels running in a radial direction from the 
periphery to the pupillary border, which are enclosed in a 
thick adventitia and surrounded by loosely arranged cells 
which fill up the space between them. 

On the posterior surface of the iris is a continuation of 
the retinal pigment layer which makes the iris absolutely 
impervious to light except that admitted through the pupil. 
The posterior surface also contains numerous elastic fibres 
which run radially from the ciliary border to the pupil, and 
by the contraction of which the pupils dilates. The natural 
position of rest of the iris, then, is with the pupil semi- 
dilated. At the pupillary margin is the thin flat band of 
muscular tissue which causes the pupil to contract, called 
the sphincter iridis. 



THE EMMETROPIC OR IDEAL EYE. 43 

The Refracting Media of the eye have their character- 
istic density and refraction index as given in the table 
below from Foster : — 

Water 1.3333 

Aqueous 1.3366 

Vitreous 1.3394 

Lens Periphery 1.39 

Lens Nucleus 1.43 

Cornea 1.3333 

Kays of light, in reaching the retina, traverse in suc- 
cession the following surfaces and media : the anterior 
surface of the cornea ; its substance ; its posterior surface ; 
the aqueous humor; the anterior surface of the lens; its 
substance; the posterior surface of the lens; and the 
vitreous humor; so that we have, including the air, four 
surfaces and four media. 

The air as one of the media is not to be overlooked, for 
the difference in density between it and the cornea is a 
powerful element in the refraction of rays, and if we sub- 
stitute water for it by placing the eye beneath the surface, 
its refracting power is much reduced. 

This constitutes a rather complicated system which, 
for purposes of study, may be simplified somewhat. For 
instance, the cornea is not absolutely homogeneous in its 
structure and its anterior surface is not absolutely parallel 
with the posterior one; but if we consider them as parallel 
and the substance nearly homogeneous and having the same 
refracting power as the aqueous, it is evident that the 
cornea may, without serious error, be considered as consist- 
ing of one surface, the anterior, separating two widely 
differing media — the external air and the aqueous. 

In the same way, without fear of serious error, we may 
disregard the varying refraction of the concentric layers of 



44 REFRACTION AND MOTILITY OF THE EYE. 

the lens and consider it as a homogeneous substance 
bounded by an anterior and a posterior surface. 

We have, therefore, to deal with three surfaces and 
three media: 

First, the anterior surface of the cornea, where rays 
of light passing from the thin air into the denser and 
practically homogeneous medium afforded by the corneal 
substance and the aqueous, are very powerfully refracted. 

Second, the convex anterior surface of the lens, 
where the rays pass from the aqueous into the more re- 
fractive lens. 

Third, the posterior surface of the lens where the rays 
pass into the less refractive vitreous. 

The surface of the cornea and the two surfaces of the 
lens are all approximately centred on a line which passes 
perpendicularly through the centre of the cornea, the centre 
of the lens and the centre of the fundus or back part of 
the eye. 

This imaginary line is called the optic axis (Fig. 32). 
Some authorities insist that it passes through the fovea 
posteriorly, while others argue that it passes a little above 
and to the nasal side, but we shall find later that the 
position of the fovea varies somewhat in different eyes. It 
has, however, been shown mathematically that a complex 
optical system of no matter how many surfaces and media, 
if centred on one axis, may be treated as though it con- 
sisted of their equivalent in a single surface. 

Such an eye in which the refracting media are re- 
placed by one refracting surface of such a strength that 
parallel rays are brought to a focus on the retina is. called 
the "Keduced Eye" (Fig. 32). Its refracting surface must 
necessarily have a sharper curve or a higher index of refrac- 
tion than the human cornea. 



THE EMMETROPIC OR IDEAL EYE. 45 

The cardinal points of such an eye are as follows : the 
principal point P, where the imaginary surface cuts the 
optic axis, the nodal point N, the posterior principal focus, 
which would fall on the retina and the anterior principal 
focus, where parallel rays emerging from the eye would cut 
the optic axis. 

By aid of this simplified or reduced eye we are enabled 
to trace out the paths of light and study the formation of 
images on the retina. 




Fig. 32. 

Let Fig. 33 represent an eye in which, while the cornea 
and lens are shown, all refraction takes place at the sur- 
face, indicated by the line passing through the optic axis 
at P, representing an imaginary medium which shall be 
the equivalent of those actually present. Then P will be 
the anterior principal point and N the nodal point. The 
formation of the retinal image is exactly like that thrown 
on a screen by any convex lens. Rays of light from the 



46 REFRACTION AND MOTILITY OF THE EYE. 

point A pass through the nodal point unrefracted to A! , 
where they are joined by the other rays from A which have 
passed through other portions and have been refracted. A 
and A' are therefore conjugate foci, as are B and B' . The 
points between A and B have their corresponding conjugates 
between A' and B' ', and therefore A'B' constitutes the image 
of A B formed on the retina which is evidently a real image 
and inverted. Hence, supposing the eye to be in an optical 
condition, in which a distinct image of the arrow is formed 
on the retina, we can find the position of the tip of the 




Fig. 33. 

arrow on the retina by drawing a straight line from the tip 
of the object through the nodal point, while the image of 
the notch of the arrow and of the intermediate points is 
found in the same way by drawing straight lines through 
the nodal point. 

Visual Angle. — The visual angle is the angle formed 
by lines drawn from the extremities of the object of regard 
through the nodal point of the eye, or the angle which the 
object subtends at the nodal point of the eye (Fig. 34). 

Evidently, if the object is nearer the eye, the visual 
angle is greater, and if further away, it is smaller. 

The size of retinal images corresponds to the visual 
angles at which they are seen. Therefore, objects having 
the same visual angle will have equal retinal images and 
appear the same size. 



THE EMMETROPIC OR IDEAL EYE. 



47 



This is modified somewhat as the result of experience. 
For instance, a row of telegraph poles appears to diminish 
in size as their distance increases, because their visual 
angle becomes less, but we know them by experience to be 
of similar size, which leads our judgment to rectify the 
erroneous impression of our vision. 




Fig. 34. 



Minimum Visual Angle. — This is the smallest angle 
at which objects can be seen and identified, and is also 
spoken of as the limiting angle of vision. This varies con- 
siderably in different individuals. Those who are young 
and with strong nerve perception, can see objects under a 
smaller visual angle, or in other words, when the object is at 
a greater distance. Xaturally, too, the visual angle will 
vary somewhat in the same individual under different con- 
ditions of illumination, atmosphere and retinal fatigue. It 
is necessary to have some fixed standard by which we can 
measure the visual acuity of patients, so as to know 
whether it is above or below the normal, and to be able to 
inform ourselves later whether it has increased or grown 
lees. Such a standard is afforded by the average visual 



48 REFRACTION AND MOTILITY OF THE EYE. 

acuity of the emmetropic eye. It has been found experi- 
mentally that all objects which subtend a visual angle of 
five minutes are distinctly visible to the normal eye, while 
if the angle is less than this, they become indistinct. 
Based on this, we have a series of test letters and objects of 
such size that at stated distances they subtend angles of 
five minutes vertically and horizontally. The ordinary test 
card is a good example. It contains at the top a letter or 
figure so large that at a distance of two hundred feet, or 61 
metres, it subtends an angle of five minutes, A second, 




Fig. 35. 

somewhat smaller, series subtends the same angle at one 
hundred feet (thirty metres); another at seventy feet 
(twenty-one metres), and so on down to a series of very 
small letters at ten feet (three metres). Evidently, these 
all subtend the same angle and, consequently, the eye which 
can distinguish the small letters at ten feet, can distinguish 
equally well the larger ones at two hundred, and vice versa. 
Accommodation. — We have seen that rays of light 
coming from a source so distant that they are practically 
parallel, are so bent by the ideal or emmetropic eye as 
to come to a focus exactly on the retina and forming a 
distinct image, make distant vision possible. Evidently, 
if the rays come from a source close at hand, they will be 



THE EMMETROPIC OR IDEAL EYE. 



49 



divergent and will be caught by the retina before they come 
to a focus. 

Let us consider for a moment with a simple convex 
lens and a screen the effect of such rays when intercepted 
away from their focus. (Fig. 36). Rays of light from a 
point after passing through such a lens, are collected in 
the shape of a cone of light, and if the screen be placed 
exactly where the rays .cross at the apex of the cone, a 
point is formed on the screen which is the exact image of 
the original source of light. If the screen is moved toward 



O : O 




the lens, the appearance on it is that of a circle of light 
formed at the place where the screen truncates the cone, 
and the circle becomes larger and less distinct the further 
from the focal point it is formed. The same thing takes 
place, if the screen be moved in the opposite direction 
beyond the focal point, where the rays after crossing have 
become divergent. Exactly the same thing takes place in 
the human eye, when rays enter which would come to a 
focus behind the retina, and a circle formed on the retina 
instead of a point. 

If the rays come from two distinct, separate points, the 
image formed consists of two circles which may overlap 



50 REFRACTION AND MOTILITY OF THE EYE. 

each other and give the impression of an indistinct oval. 
In the same way a line may be considered as composed of 
a number of points, each of which under these circumstances 
appears as a circle, the final result being of a broad band 
with oval ends and indistinct outline". Every visible object 
is made up of imaginary points, which must have their 
conjugates focus exactly on the retina to form a distinct 
image of the whole object, and if part or all of these points 
give the impression of circles, distinct vision is impossible. 
These circles are called diffusion circles and are formed on 
the retina in every one of the various anomalies of refrac- 
tion, and it is to prevent their formation or to relieve the 
eye of the task of preventing them, that we prescribe glasses. 

In the photographic camera these diffusion circles may 
be obviated in three ways : by changing the distances 
between the lens and the ground glass by moving one or 
both ; by moving the object further away from the camera ; 
or by increasing the strength of the lens. In the living eye 
some of the lower animals can increase their refractive 
power by pushing the lens backward and forward at will; 
but man has choice only between the last two alternatives 
and increases his refraction by increasing automatically 
the strength of his refracting media, and when this fails in 
old age, resorts to pushing the object away from his eyes. 
This change in refraction takes place chiefly in the crystal- 
line lens through a physiological process of accommodation, 
to understand which it may be best to review briefly the 
anatomy of the tissues. 

In the first place, the lens is composed of tissue which 
is elastic and tends to assume a spherical shape, when 
unrestrained. When the eye is at rest, however, many 
experiments show that the lens is very much flattened, 
especially its anterior surface, and this is supposed to be 



THE EMMETROPIC OR IDEAL EYE. 51 

due to the tension of the fibres of the zonula which spring 
from the ciliary processes and chorioid, pass forward on 
the inner surface of the ciliary body and split into two 
layers, one of which becomes continuous with the anterior 
capsule of the lens and the other with the posterior one. 
When these fibres are tense, the lens becomes oval in its 
cross section, and when they are relaxed, the cross section 
is a circle. This relaxation is effected by the ciliary body 




Fig. 37. 

or muscle, which is an annular tissue attached all the way 
round just behind the junction of the cornea and sclera and 
with longitudinal fibres running back to inosculate with the 
chorioid at the ora serrata. 

When the ciliary muscle contracts, the longitudinal 
fibres pull the chorioidal tissue toward the fixed point of 
the muscle at the corneal margin, and in doing so, also pull 
forward and relax the fibres of the zonula which are inter- 
woven with the muscular tissues. The main factor, how- 
ever, is probably the annular portion of the muscle which has 



52 REFRACTION AND MOTILITY OF THE EYE. 

a sphincter-like action, and in contracting the size of the 
ring produces a complete relaxation of the zonula. It is for 
this reason that individuals who have to use the accommo- 
dation unduly have a hypertrophy of the ciliary muscle and 
especially its circular fibres. 

As the surface of the lens assumes a more curved 
shape, there is necessarily produced a corresponding 
diminution in its equatorial diameter, the equator con- 
stantly receding toward the axis of the eye and keeping 
a fixed distance from its ciliary muscle as it contracts. 

The increase in curvature affects both surfaces, but 
especially the anterior, which advances, while the posterior 
does not materially change, its position in the fossa 
patelliformis. By this central forward-bulging of the lens 
the iris is pushed forward in the centre, while it recedes 
slightly on the periphery, and at the same time there is a 
contraction of the pupil. 

In order to measure the increase in power of the 
lens in the act of accommodation, we must determine two 
. points. One of these is the so-called far point, for which 
the eye is focused when completely relaxed, which in the 
emmetropic eye is infinity ; and the other the near point, 
or the nearest point at which distinct vision is possible by 
exerting the entire accommodation. 

The far point is determined by testing at a distance of 
twenty feet with the trial card, in which case the 
emmetropic eye would have a vision of 20/20. The near 
point is determined by bringing fine type closer and. closer 
to the eye, till distinct vision ceases. The distance between 
the near and far points is the region of accommodation, 
while the amplitude, or range, or power, of accommodation 
-is the difference between the refraction of the eye at the 
far and near points in dioptres. 



THE EMMETROPIC OR IDEAL EYE. 53 

For instance, in the emmetropic eye which is adapted 
to parallel rays, if the object of regard be moved up to a 
distance of one meter, it can be seen by the interposition 
of a lens of ID., or by accommodating an equivalent 
amount. The object at ten inches or a quarter of a metre 
could be seen by aid of a 4 D. glass or by accommodating, 
and if the object cannot be brought nearer to the eye with- 
out blurring, it is evident that the near point is at ten 
inches while the amplitude or amount of accommodation is 
4 D. If the near point of the eye were at five or four or 
two inches, the amplitude would be 8 D. or 10 D. or 20 D., 
respectively. 

The power of accommodation is temporarily abrogated 
by the use of atropin, and also varies greatly with the age 
of the individual. It is greatest in extreme youth and 
diminishes regularly with age, until it disappears in old age 
altogether. This loss of power is the result of a physio- 
logical diminution of the elasticity of the lens substance 
which comes with age. Xo matter how powerful the 
action of the ciliary muscle, it can only permit the lens to 
assume the shape caused by its internal elasticity, and as 
it becomes hardened and inelastic, it fails to respond and 
remains constantly flat, causing the near point to recede. 
The state of the accommodation at different ages is well 
shown in the following diagram from Donders, which, how- 
ever, shows only average measurements from which there are 
many and wide variations (Fig. 38), as shown by Duane. 

Thus, at ten years the emmetrope has a near point of 
7.1 cm. equal to 2% inches, and the accommodation is 
equal to a lens of 14 D. At 20 his near point has receded 
to 1" cm. or four inches and his power is 10 D., while at 45 
his near point is nine inches and his power has fallen to 
4.50 D. After that he has difficulty in reading fine type 



54 



REFRACTION AND MOTILITY OF THE EYE. 



without holding it further and further from his eyes and 
soon has to bring his near point up by wearing reading 

glasses. 

ACTIO 15 20 25 30 35 4Q 4-5 SO SS 60 65 70 75 60 



^ 

o 10 



s 7 

o 

I* 

rf 

S 2 

2 i 







\ 


3 


— W 


V. 


\ 


A^ ; 


H 


\ 


\ 


^ 


\? 


r t ^ 


V 


s 







"I 

12 53 



"i 



25 * 

50 % 
100,3 



100 



50 



Fig. 38. 



The examination of even a normal eye includes the 
testing of a number of different functions, and it is advis- 
able for the student to get the habit of making the examina- 
tion in a regular and orderly manner, making a careful 
permanent record of the facts as ascertained. This record 






THE EMMETROPIC OR IDEAL EYE. 55 

will be very much more valuable, if it contains a history 
of the patient, not only the eye history and present eye 
symptoms, but the family and personal medical history, 
making special note of diseases which may affect the 
integrity of the eyes, and also of many functional troubles 
which at times result from ocular imperfections. A very 
convenient way of keeping such records is by means of a 
card index, or better still, a loose leaf system, the history 
being recorded on one side at such length as seems desirable, 
while on the other side is the examination record. The 
family history should include illness and eye diseases 
which might be hereditary, especial note being made of 
conditions like migraine, epilepsy, neurasthenia, which 
might be either dependent on eye conditions or indicate 
a hereditary nervous instability and so make otherwise 
unimportant ocular defects an overburden. 

The personal history should include data of age, 
occupation, whether taxing to the eyes or not, of habits 
which may affect the eyes such as the use of alcohol, 
tobacco and drugs, of all severe illnesses which might have 
left diseased eye conditions, and especially of recent ill 
health which might cause a temporary ocular insufficiency. 
The nervous diseases, such as neurasthenia, hysteria, 
epilepsy, locomotor ataxia, neuralgia, should be recorded, 
and especial attention paid to headache, its frequency and 
severity, whether it is hereditary like migraine with nausea 
and scintillating scotomata, whether it occurs daily or only 
occasionally, whether it appears to be caused by use of the 
eyes and relieved by their disuse, whether near or distant 
vision is the most trying and whether it is a frontal or an 
occipital. Inquiry should be made as to digestion, circula- 
tion, etc., particularly as regards functional disorders. 

The data regarding the eyes themselves should include 



56 REFRACTION AND MOTILITY OF THE EYE. 

all the symptoms of which the patient is aware, such as 
pain, photophobia, failure of vision, whether for near or 
far, the existence of scotomata, diplopia, and also a record 
of the objective peculiarities, such as inflammation, mal- 
formation of the lids, inequality of the pupils, strabismus, 
etc. 

The physician then proceeds to record the following 
facts : — 

1. The distant vision in each eye, with and without 
glasses, indicating whether a cycloplegic was used or not. 

2. Ophthalmoscopic examination, including reaction 
of the pupils, both singly and consensually, the transparency 
of the various media, the conditions of the fundus and an 
estimate of the refraction. The use of suitable rubber 
stamps will facilitate these records. 

3. Eetinoscopic examination of refraction. 

4. Ophthalmometry measurement of corneal curve 
and radius of curvature. 

5. Near vision : measurement both of visual acuity 
and amplitude of accommodation, whether normal and 
sufficient for needs or not. 

6. Motility of the eyes. Relative position of the 
optic axes in state of rest. Involuntary or fusion power. 
Voluntary or rotatory capacity. 

7. The field of vision. Peripheral retinal sensitive- 
ness. 

8. Color sensation. 

We have seen that the average eye can perceive clearly 
objects which subtend a visual angle of at least five minutes 
and our test cards for estimating the visual acuity are 
based on this fact. They generally contain, as shown in the 
illustrations, a series of letters of different sizes, which 
subtend this angle at different distances varying from two 



THE EMMETROPIC OR IDEAL EYE. 57 

hundred feet or sixty-one metres for the largest down to 
eight feet or 2.5 metres for the smallest. These test types 
are generally black on a white background, but sometimes 
the background is cream-colored, which is softer to the eye. 
Cards are made also in which the letters only are white, 

E 

C B 

D L N 

P T E R 

F Z B D E 

OPLCTG 

AFEOBFDZ 

KFBTVZBDrSO 

VITAOBSlirli 

Fig. 39. 

while the background is black. For normal eyes the visual 
acuity is usually slightly greater when tested on the first 
variety, but eyes which are tired and strained or inflamed, 
with some photophobia, will frequently appear to much 
better advantage with cards which do not reflect so much 
light. 

There are many test objects more scientific and accu- 
rate than letters and numbers, some of which can be dis- 



58 REFRACTION AND MOTILITY OF THE EYE. 

tinguished at considerably greater distances than others of 
the same size, but in the consultation room the letters have 
proven far more convenient and time saving than any so 
far suggested. 

For children cards have been devised with kinder- 
garten objects of a size corresponding to the ordinary 
charts, while for illiterates numerals or the letter E with 
the prongs pointing in different directions are made, the 
patient being expected to indicate by gestures the direction 
in which they point. 

The selection of test cards is a matter of individual 
choice, but the office should be provided with several to 
avoid the memorizing into which patients, especially 
children, unwittingly fall. 

Tests of distant vision should preferably be made at a 
distance of twenty feet or more, but when this cannot be 
readily obtained, the space can be doubled by having the 
patient read the letters from a mirror across the room as 
they are reflected from a card beside him, when the apparent 
distance is equal to the sum of both incident and reflected 
rays. Owing to the lateral inversion, a special card with 
letters reversed laterally is provided. These tests can be 
made by a good day light, but it is generally much better 
to have the test cards artificially illuminated, as much 
greater uniformity is secured in this way. Bach eye 
should be tested separately, the other being screened, but 
not closed, and as the patient always sees better with one 
eye when he is also using the other, it is often better to 
use, instead of a screen or disc, a strong convex lens. The" 
patient is then using both eyes, but all that he sees dis- 
tinctly is seen with the eye which is being tested. 

Each letter on the cards has marked over it in both 
feet and metres the distance at which it should be read, 



THE EMMETROPIC OR IDEAL EYE. 59 

and the record of acuity will consist of a fraction, the 
numerator of which is the distance at which the patient 
sat from the card, and the denominator the size of the 
smallest row of letters correctly read. These fractions 
being records of actual conditions are better left unreduced. 
If the patient reads only part of a line, the number of 
letters missed should be indicated by asterisk or question 
mark for each. If at the usual distance the patient can 
see no letters on the card, he must be allowed to approach 
till he can read, and the distance now becomes the numera- 
tor of the fraction on record. Patients, who are incapable 
of perceiving letters, may be tested as to their ability to 
count fingers at a maximum distance, which is carefully 
noted. Eyes not even capable of distinguishing form in 
fingers, may be tested by their ability to perceive light 
from a candle or a mirror. 

It is advisable for the student to get into a regular and 
unvarying method of procedure both in the examinations 
and recording of the results, as he is thus very much less 
likely to overlook facts or make mistakes. The abbrevia- 
tions K. E. or 0. D. (oculus dexter) are used for right eye, 
and L. E. or 0. S. (oculus sinister) for left eye. 

Vision 0. D. 20/20; 0. S. 20/200 means that the 
patient was seated at a distance of twenty feet from the 
card and with the right eye he saw the letters which he 
should see at that distance, while with the other eye for 
some reason he could only see the letters which should have 
been visible at 200 feet. The same record expressed in 
metres would have been : — 

E. E. K/6 or VI/VI ; 

L. E. 6/fiO or VI/LX. 

The visual acuity with appropriate glasses is also part 
of the record, as follows : — 



60 REFRACTION AND MOTILITY OF THE EYE. 

For instance, a patient who accepted a certain glass 
with or without improvement, would have his vision 
recorded thus : — 

K. E. 20/70 ?.?■+' »C + 1 ax. 90 = 20/30 

L. E. 20/200 — 2 ax. 1.80 = 20/30. 

To determine the near point, we employ small cards on 
which are printed letters of various sizes, ranging from 
very small up to moderately large letters. The first cards 
for this purpose, devised by Jaeger, were entirely empirical 
being printed from type of various sizes found in every 
printing office. Later on, Snellen introduced test type 
on the same principle as his large wall types, each letter 
subtending a visual angle of five minutes at a given dis- 
tance. Above each series of letters is printed the distance 
at which it should be normally visible, from twenty-five 
centimetres (ten inches) up to two hundred centimetres. 

The patient should be seated with a good light falling 
on the card from a source over his shoulder. With each 
eye separately he then picks out the finest type which he 
can see distinctly and approaches the card toward the eye 
till the letters begin to appear hazy. The distance between 
this point and the eye is carefully measured in the metric 
or the inch system, and the record entered as follows : — 

Near V. = 0.5 D. at 25 cm., 
or less scientifically, but approximately: — 

No. 1 Jaeger at ten inches. 

Very often, as we shall see in the chapter on "Pres- 
byopia/' the patient cannot read fine print without glasses, 
when we record the glass selected, the size of type it en- 
abled the eye to perceive and the nearest and farthest point 
at which vision was distinct. For instance, 

with 2.50 sph. No. 1 Jaeger 10-15 inches 

or Snellens .50 D. 25-37 cm. 



THE EMMETROPIC OR IDEAL EYE. 61- 

When patients are illiterate the same purpose is sub- 
served by the hair optometer (Fig. -10), which consists of a 
small frame across which are stretched several hairs or fine 
threads. The patient approaches the instrument to the 
eyes till the strands cease to be distinct, when the distance is 
carefully measured by a tape attached to a hook on the 
handle. 

The use of the trial case will be taken up more fully in 
chapters dealing with various^ forms of ametropia, but a 
description of the contents of the trial case and its purpose 
may not be amiss. Each case contains a series of convex 




Fig. 40. 



and of concave spheres running from .25 D. up to 20 D. by 
varying graduations, and of convex and concave cylinders 
from .25 D. to 5 D. As a rule, the more minute the sub- 
divisions of the dioptres from .25 to 3, the more useful the 
case, In the convex glasses, for instance, whether spherical 
or cylindrical, one is constantly making use of fractions of 
the first dioptre, while the stronger lenses are less fre- 
quently called for, and the strongest only occasionally. 

To a less extent the same thing is true of the concave 
lenses. Each case also contains a set of prisms, varying in 
strength from y 2 A to 20 A > as well as a series of colored 
and smoked plain glasses and perforated disks, whose use 
will be more fully explained later on. Especially impor- 
tant is the trial frame, which should be light, but not flimsy, 
capable of adjustment to suit various types of features and 
of being firmly held in this position. 



CHAPTER III. 

OBJECTIVE EXAMINATION OF THE EYE— 
OPHTHALMOSCOPY. 

We have seen that the emmetropic eye is one in which 
parallel rays of light come to a focus on the retina when 
the ciliary muscle is completely relaxed, that it is capable 
of seeing distinctly ail objects, whatever their distance, 
which subtend a visual angle of five minutes, and that it 
has a definite average power of accommodation depending 
on the age of the individual. 

The absolutely emmetropic eye is one of the rarest 
things in nature. It is to be distinguished from the far- 
sighted or hyperopia eye by which parallel rays are not 
refracted strongly enough and consequently come to a 
focus behind the retina; the) nearsighted or myopic eye by 
which parallel rays are too strongly refracted and come to 
a focus in front of the retina; and the astigmatic eye in 
which the refraction in different meridians varies so that 
either one or both fail to focus on the retina because of over- 
or under-refraction. In one individual the vision may be 
very much below normal, while another may preserve the 
keenness of his vision by the use of his accommodation, in 
which case he is likely to suffer from his over-exertion. 

In either case it is the object of the refractionist to 
discover the .direction of the rays which do focus on the 
retina, and the lens, or combination of lenses, by aid of 
which he shall be able to focus parallel rays on his retina. 
In other words, we try to make the individual emmetropic 
by aid of glasses and so either improve his vision or relieve 
the strain on his ciliary muscle, or both, and in case his 
vision is not improved, to discover the cause : whether the 
disturbance be due to changes in the refracting media or 
to lessened perception on the part of the deeper structures. 
(62) 



OPHTHALMOSCOPY. 63 

In making this examination, too much stress cannot be 
laid on the value of system in our method, lest we carelessly 
overlook some important fact. After examining the condi- 
tion of the lids and conjunctiva, we begin the inspection of 
the refracting media by the method of : 

Oblique illumination consists in the concentra- 
tion of light on the cornea by a very strong convex lens. 




From May " On the Eye." Courtesy of Wm. Wood & Co. 

Fig. 41. 

A light is placed to one side and somewhat in front of the 
patient, and its rays are concentrated by a 13 to 15 D. lens 
into a cone of light whose apex is thrown on the cornea. 
This is sometimes called "focal illumination," since the 
point at the focus of the lens appears with great sharpness, 
especially if the examination be conducted in a darkened 
room. By this method we can recognize even very small 
opacities of the cornea which intercept the light and at once 
become visible. It is also well at times to test the sensitive- 



64 REFRACTION AND MOTILITY OF THE EYE. 

ness of the cornea to the touch, with a bit of cotton, for the 
anesthesia which is suggestive of hysteria or glaucoma. 
Throwing the cone of light on the deeper portions of the 
eye, we note the anterior chamber, particularly its depth, 
and then pass to the iris, observing whether its radiations 
are distinct and its color like its fellow. Throwing the 
cone of light off and again on the pupil, we notice whether 
the iris reacts to light and whether sharply or sluggishly, 
also when it dilates, whether the pupil is free, round and 
central, or whether it is irregular from adhesions. We also 




Fig. 42. 

note any tendency of the iris, to shake like the edge of a 
drapery, as abnormal and suggestive of a dislocated lens, 
and then investigate to see whether illumination of one eye 
causes a consensual contraction of the pupil in the other. 
Furthermore we test the reaction of the pupil to accommo- 
dation and convergence, and, lastly, note whether the pupil 
is a clear black or not. Even with the dilated pupil we 
can see very little of the lens by lateral illumination, unless 
there are opacities present. When it is perfectly trans- 
parent, we can demonstrate its presence by the presence of 
the Purkinje- Sanson reflexes. 

If in a dark room a candle be placed before and some- 
what to one side of the eye, a reflection may be seen from 
the three refracting surfaces of the eye. The one on the 



OPHTHALMOSCOPY. 65 

cornea is large and easily seen, being an upright image 
of the candle name which, when the candle is moved, moves 
in the same direction. The one formed on the anterior 
surface of the lens is upright and larger than the pre- 
ceding one, but so very faint as to be seen with great diffi- 
culty. The one formed on the posterior concave surface of 
the lens is the one on which we rely. It is very bright, but 
very small and inverted, and seems to move in the opposite 
direction to any movement of the candle. If it is absent, 
either the lens is wanting or so lacking in transparency 
that it can readily be recognized by other methods. 

In examining the interior of the eye, it is advantageous, 
though not necessary, to have the pupil widely dilated. 

The Ophthalmoscope. — The ancients very early ob- 
served that the eyes of many animals glowed in the dark 
"like coals of fire," and it was long known that the human 
eye, when placed under water, emitted light in the same 
way: hence it was supposed that the eye was the source of 
light till it was demonstrated that death makes no change in 
the phenomenon. Later it was shown that almost any eye 
in which the pupil is dilated becomes luminous under proper 
conditions and that the eye is not the source of light, but 
merely reflects the light it receives, the red color being due 
to the blood-vessels in the chorioid. The eyes of most wild 
animals are highly hyperopic, or weak in their refractive 
power, and hence emit rays of light which are widely 
divergent, and to the observer who can put himself in the 
path of any of these rays, the pupils appear luminous. 
When the. normal human eye is placed beneath the water, 
the refraction of the cornea and aqueous are practically 
nullified, and rays of light from a candle after entering the 
eye, come out widely divergent, making the pupil appear 
luminous ; but if the water be removed without changing 



66 REFRACTION AND MOTILITY OF THE EYE. 

the position of the light or the eye, the latter ceases to be 
luminous, because the rays now come out parallel, and if the 
observer places himself in their path, his head cuts off the 
source of light. In the albino, whose eye is lacking in pig- 
ment, light can enter the eye through the sclera and 
chorioid, and consequently it appears luminous, whatever 
the position of the observer. Evidently, to view the fundus 
of the normal eye, the eye of the observer must be in a direct 
line with the light without cutting it off or being dazzled by 
it. This problem was partly solved by an ancient observer 
who looked through a hollow tube held in the flame of a 
candle. Helmholtz, only sixty years ago, established the 
theory of the ophthalmoscope and made the first practical 
instrument which has passed through many stages up to the 
perfected instruments of to-day. Essentially the ophthal- 
moscope, of whatever pattern, consists of a mirror which 
reflects light directly into the. patient's eye, while the 
observer places himself in the path of those rays by gazing 
through a small hole in the centre of the mirror. 

This is sufficient for observing the fundus of the 
emmetropic eye, but when the eyes of either observer or 
observed are ametropic, a series of lenses placed behind the 
mirror gives us all the essentials of the refracting ophthal- 
moscope of to-day. Perhaps the most popular ophthalmo- 
scope, in America at least, is the Loring or one of its modifi- 
cations, which is shown in the illustration. In it the 
mirror is slightly concave so as to concentrate the light, and 
the lateral edges are cut away so that the mirror may be 
tilted to either side, which is a great help in directing the 
rays from a lamp beside the patient's head into his eyes. 
Behind the sight-hole in the mirror is arranged a revolving 
disc containing an aperture and lenses ranging from -f- 7 D. 
to — 8 D., which can be brought in turn behind the hole in 



OPHTHALMOSCOPY. 



67 



the mirror. In case of need there is over the disc a movable 
quadrant containing a + -50 and a + 16 D. and a — .50 
and a — 16 D., any one of winch can be used in conjunction 
with any lens in the disc. It is thus possible to proceed by 
half dioptres all the way from + 23 to — 24 D. 

The other ophthalmoscopes in use differ from the 
Loring type chiefly in their method of changing the 




strength of the lenses behind the mirror. Any one of them 
made by responsible parties will be satisfactory. 

The source of light is a matter of choice. It may be 
oil or gas, or electric light. If the electric bulbs are used, 
they must be of frosted or ground glass. These may be 
laid on the patient's pillow in bedside work. Several 
ophthalmoscopes are available in which the light comes 
from a tiny electric lamp, activated from a dry cell in the 
handle. They are very easy to use and are especially 



68 



REFRACTION AND MOTILITY OF THE EYE. 



adapted to bedside work in the hospital, where ordinary 
means of illumination are not convenient without making 
the patient sit up. When the patient can sit up in bed, a 
thorough examination can bet made by aid of a candle. 

The ophthalmoscopic examination should 
be made in a dark room, the patient being 
seated on a comfortable chair, and the lamp 
should be about on a level with the patient's 
eye and on a plane considerably back of it, 
so that light from it may be thrown into the 
eye without unnecessary maneuvers. The ob- 
server should be seated beside the patient and 
facing him in such a way that their eyes are 
about on a level and near enough to approach 
his eye very close to his patient's. Almost 
necessarily the surgeon must use his right eye 
in examining the right eye, and his left in 
examining the left, since otherwise his nose 
will come in contact with the patient's and 
he will not get close enough to examine 
satisfactorily. 

The Direct Method. — Placing the 
ophthalmoscope before his eye and looking 
' Fig. 43a. through a + 6 or + 7 lens, the physician 
should first throw the light into the eye from 
a distance of twelve or fifteen inches and gradually ap- 
proach his head to that of the patient. In this way he not 
only has the advantage of the condensation of light from 
the concave mirror, but when he approaches to the proper 
distance, sees all the details magnified by a convex lens. 
Turning the mirror to one side so as to throw the pupil in 
the shadow and then exposing it to the bright light sud- 
denly, he should note whether it reacts properly to the 



OPHTHALMOSCOPY. 



69 



light stimulus. The light reflected from the retina returns 
to the observer's eye, so that the whole pupil seems 
illuminated by a brilliant glow which, if perfectly clear, 
indicates that the refractive media are transparent. He 
should note whether the pupil is round, or whether its edge 
is adherent at any point to the lens, causing it to expand 
and contract unevenly, or whether any spots of pigment on 




Fig. 44. 



the lens near the margin indicate previous adhesions. If 
there are spots of this kind or opacities in the cornea, lens 
or vitreous, they will interfere with the return of light to 
the eye of the observer and will appear as black spots or 
ma^es against the red background. Then, while watching 
these closely, the patient is directed to move his gaze up or 
down, and the surgeon notes carefully their position rela- 
tive to the margin of the pupil and can tell quite accurately 
on which of the refractive media they are situated. 

Suppose, for instance, an opacity that when the patient 



70 



EFFRACTION AND MOTILITY OF THE EYE. 



looks straight ahead appears like a black point in the 
centre of the pupil. When the eye turns upward, it moves 
wheel-fashion round a centre of rotation, and objects in 
front of the pupillary plane will seem to move upward 
while those behind will have a corresponding motion down- 
ward. If the opacity is in the cornea, it will appear to 




Fig. 45. 



move in the direction in which the eye is turning toward 
the margin of the pupil and will finally pass out of the 
illuminated pupil in front of the iris. If it is in the 
anterior chamber, it will move in the same direction, but 
being nearer the pupil, will not move so fast nor so far. 

If on the anterior surface of the lens, which is prac- 
tically in the same plane as the iris, the opacities will 
maintain the same relative position in the pupil, whichever 
way the eye may move. If they are in the substance of the 
lens or on its posterior surface, which are in planes behind 
the iris, they are, as it were^ behind the hub of the wheel, 



OPHTHALMOSCOPY. 71 

and, when viewed from the front, seem to move in the 
opposite direction to the motion of the eye. If the 
opacities are in the vitreous, they move down when the 
eye moves up, and vice versa, and their motion is apparently 
more rapid, as they are further from the iris and nearer to 
the posterior pole of the eye. The result is the same when 
the patient keeps his eye quiet and the observer moves from 
one position to the other. 

Opacities can also be localized by their relation to the 
corneal reflex. An imaginary line draAvn from the sight 
hole of the ophthalmoscope through the light reflex seen on 
the cornea, must be perpendicular to its surface and so pass 
through its centre of curvature, which we know to be in the 
back part of the lens. An opacity which always appears 
very close to the light reflex when viewed from several 
directions must be near this centre of curvature. If it in- 
variably seems nearer the centre of the cornea than the re- 
flex it must be proportionately further forward, while if it 
seems invariably outside the reflex it must be further back. 

It is not intended in a work of this kind to consider 
the various causes of opacity in the refracting, media, but 
having ascertained that there are no opacities to obstruct 
a view of the fundus, the student should — looking now 
through the aperture — approach his eye to that of the 
patient, being careful meanwhile not to lose sight of the 
reflex. When the light from the ophthalmoscopic mirror 
is thrown on a screen or on the patient's forehead or cheek, 
it forms an oblong of condensed light, in the centre of 
which will be seen a dark spot which corresponds to the 
aperture in the mirror. If the ophthalmoscope be moved 
in such a way that the dark spot coincides with the pupil of 
the patient and approaches as closely as possible, a good 
view of the fundus will be obtained if both eyes be emme- 



72 REFRACTION AND MOTILITY OF THE EYE. 

tropic and with accommodation relaxed. The beam of 
light thrown into the patient's eye is reflected from the 
retina, and if the eye be emmetropic and completely 
relaxed, emerges in parallel rays. A central bundle of 
these rays passes through the aperture in the mirror and 
entering the eye of the observer should come to a focus 
exactly on his retina, giving a clear picture of the fundus. 
But here a difficulty confronts the beginner, for conscious 
that he is looking at an object very close to his own eye, he 
insensibly accommodates and the rays focus before reaching 
his retina, and he sees indistinctly. The ability to relax his 
own accommodation is the basis of all successful work with 
the ophthalmoscope, and if the student will exercise 
patience and try to imagine that he is looking at an object 
far from him, at the same time learning to keep both eyes 
open, he will gradually learn to relax his ciliary muscle. He 
should use every opportunity to examine eyes with dilated 
pupils and can help himself very much by practicing with 
an artificial eye. Till he does learn to relax, he can get a 
clear view of the fundus only by turning in front of the 
aperture concave lenses of three or four dioptres, which 
will overcome the effect of his accommodation. 

It simplifies matters greatly to have the patient fix his 
gaze on some object directly in front and on the same level, 
lest he keep his eye roving from one object to another, and 
with the eyes in this position the light from the mirror 
should fall on the important structures at the back of the 
eyeball. If the observer does not catch sight of the nerve 
head at once, he can readily find it by following up the 
course of one of the blood vessels. In this direct method 
of examination the image formed in the eye of the observer 
is real, erect and magnified, the size of the image depending 
on (1) the refraction of surgeon and patient, (2) the dis- 



OPHTHALMOSCOPY. 73 

tanee between their eyes, and (3) if a glass be necessary, 
the distance of the glass from the eye of the patient. Cal- 
culated from emmetropic eyes without lenses, the magnifi- 
cation approximates fifteen diameters. 

It cannot be emphasized too strongly that the student 
should employ every possible opportunity to examine the 
normal fundus, not only that he may acquire the proper 
facility in the use of the ophthalmoscope, but that he may 
familiarize himself with the numerous variations which 
are still within the limits of the normal, and to this end he 
should, to begin with at least, employ a routine method of 
examination which may be very profitably supplemented by 
attempts at drawing what he sees. This not with the idea 
of acquiring skill in producing fundus pictures, but of 
cultivating the powers of accurate observation. 

The most striking feature of the fundus is the disc, or 
nerve head, or papilla, where the optic nerve and the retinal 
vessels enter the eye through the sclera. This is situated 
somewhat to the nasal side of the posterior pole and comes 
easily into view when the patient is looking straight before 
him. This opening in the sclera is not a complete one, 
since the inner layers of the sclera pass directly over it, 
forming a septum which is pierced by a number of openings 
for the transmission of bundles of nerve fibres, and is hence 
called the lamina cribrosa. In piercing the lamina, the 
fibres of the nerve lose their sheaths and become almost 
transparent. Hence the color of the nerve head is the 
glistening white of the sclera, modified by the grayish trans- 
lucent fibres of the nerve, and in the centre the openings 
in the lamina can often be seen as small gray dots, in the 
midst of the lighter lamina (Fig. 46, ^4). 

Commonly the hear! of the nerve is perfectly flat so as 
to lie in the same plane as the retina, but very often the 



74 



REFRACTION AND MOTILITY OF THE EYE. 



fibres of the optic nerve begin to separate before reaching 
the level of the retina, so that a funnel-shaped depression is 
produced, from which the central vessels of the nerve 
emerge. This is the normal vascular funnel which may be 
quite extensive without being pathological, and is called 
the physiological excavation. (Fig. 46, B.) It is not to 
be confounded with the saucer-shaped excavation in atro- 
phy (C) of the nerve or the bowl-shaped excavation in 
glaucoma (D). 




Fig. 46. 

The optic nerve is not always circular in shape, but 
may be oval with its long diameter in either direction, and 
the border is formed by a delicate rim, over which the 
vessels run as they pass out of the excavation. Sometimes, 
but not always, there is running entirely around the nerve 
head a narrow band of white Corresponding to the edge of 
the scleral fufmel which has not been covered by chorioid. 
This is called the scleral ring, and at times just outside this 
is a ring, partial or complete, of black pigment from the 
chorioid, which is called the chorioidal or pigment ring. 



OPHTHALMOSCOPY. 75 

The retinal blood vessels arise from the deepest part of 
the excavation and are distributed to various parts of the 
retina, giving off numerous branches. While there is a 
general uniformity in their course, there are many normal 
variations. As a rule there is a central artery of the retina 
which almost immediately on emerging from the nerve 
head divides into two branches, one of which runs upward 
and the other downward, each of these dividing near the 
margin of the disc into a nasal and a temporal branch. 
These four arteries divide and subdivide, and so supply the 
retina, and each is accompanied by a vein, the arrangement 
of veins corresponding very closely to that of the arteries. 
Occasionally the branching of arteries takes place in the 
nerve substance itself, and we have two or even four arteries 
instead of one, but their general distribution is unchanged. 
At times there is seen a small artery springing from the 
temporal edge of the disc and running outward. This is 
a branch of the ciliary artery and is known as the cilio- 
retinal. The arteries are to be distinguished from veins 
by being somewhat narrower, straighter, and in the larger 
vessels having a central white stripe or reflex. The smaller 
vessels can not be distinguished. It is to be noticed that the 
region of the macula to the temporal side of the disc is not 
traversed by any visible vessels, but is evidently richly 
supplied by capillaries from all the vessels which arch 
about it (See Fig. 31). 

Pulsation can often be observed in the larger vessels, 
where they make sharp bends. A venous pulse is physio- 
logical and can be produced by increasing the intraocular 
tension by pressing with the finger. Arterial pulsation is 
pathological. 

Though we can see the retinal vessel's plainly, the 
retina itself is transparent and invisible except that we 



76 REFRACTION AND MOTILITY OF THE EYE. 

can sometimes see radiating from the nerve head a fine 
grayish glistening reticulation of nerve fibres which is soon 
lost in the retina. Occasionally some of these nerve fibres 
retain their sheaths which are opaque and appear glistening 
white against the red fundus. This is regularly seen in 
the rabbit's eye. 

The rich red glow which characterizes the fundus out- 
side the nerve head is due to the blood circulation in the 
numerous capillaries of the chorioid, though, as a rule, no 
individual vessel in the chorioid can be distinguished, since 
they are covered by a layer of pigment epithelium which 
obscures them. The amount of pigment present depends 
largely on the complexion of the individual. In the highly 
pigmented individual the spaces between the chorioidal 
vessels are so packed with pigment that the vessels them- 
selves stand out by contrast as bright red striae running 
everywhere and anastomosing with each other. In the 
albino, on the other hand, where pigment is entirely want- 
ing, the chorioidal vessels are distinctly visible on a very 
pale background. The retinal vessels can be traced running 
over them, while the chorioidal ones are broader and deeper 
down, without reflexes, and by their anastomoses form a 
dense network. 

The macula lutea, or yellow spot, is situated to the 
outer side of the nerve head and at a distance of about three 
diameters of the nerve head. The best way to see it with 
the ophthalmoscope is to find the outer edge of the nerve 
and then sweep a horizontal path with the light from the 
mirror till the macula is seen. This is much easier if the 
lamp be moved away from the patient's head at the same 
time, or the patient may be directed to look at the observer's 
ear with his other eye, during which maneuvre his macula 
ought to be presented directly to view. It is the most 



OPHTHALMOSCOPY. 77 

difficult part of the fundus to see distinctly because of a 
central corneal reflex, which long embarrasses the inex- 
perienced observer. The macula lies in a region of great 
vascularity and is itself especially vascular, since it is a 
deeper red than the surrounding fundus. Surrounding the 
macula there is often a small circular ring of light, 
evidently a reflex, and occasionally a delicate stippling of 
the eye ground of the macula can be distinguished. 

In even- examination of the fundus, spots and areas of 
light are reflected from portions whose shape is suitable, 
and are recognized as reflexes by their peculiar glistening 
appearance and by their changing their shape and position 
with the movements of the ophthalmoscopic mirror. 

The observer, having discovered no opacities in the 
media and having examined the eye ground, the blood 
vessels, the nerve head and especially the macula without 
finding any abnormalities, is now ready to estimate the 
refraction of the eye. He must bear in mind, however, 
that this, while a good rough test for considerable errors, 
is not a reliable means of estimating small ones, especially 
the low degrees of astigmatism which are so important from 
the clinical standpoint. To secure even approximately 
trustworthy results, the accommodation of both observer 
and patient must be relaxed. The best way to secure this 
relaxation on the part of the patient is to have him gaze 
at some distant object on a level with and directly in front, 
which has the additional advantage of keeping his eyes 
quiet. If allowed to fix some object near at hand, he is 
very apt to accommodate in proportion to its propinquity. 
A cycloplegic must be used, if necessary. 

The rays of light reflected from the concave mirror 
enter the eye convergent and cross before reaching the 
retina, but it must be remembered that the direction of the 



78 REFRACTION AND MOTILITY OF THE EYE. 

entering rays is of no importance. The illuminated area 
of the retina, by diffused reflection, serves as a new source 
of light and therefore the direction of the emergent rays 
alone has to be taken into account. 



Fig. 47. » 

If the eye of the patient be emmetropic, the rays which 
come back to the observer through the aperture in his 
mirror must be parallel and consequently come to a focus 
on his retina, and he sees the fundus of the patient's eye 
distinctly. If, without removing the ophthalmoscope, he 
turns a + 1 D. lens into the aperture, the parallel rays from 
the patient's eye are brought to a focus in front of the 
retina of the physician, and he can no longer see distinctly. 
If, however, he turns on a — ID. glass, the focus is behind 
his retina and he can still see distinctly by accommodating 
1 D. If he uses a — 2 or 3 or 4 D. glass, he can still see 
by using a stronger accommodation till he reaches its limit, 
when objects for the first time become indistinct. 

Evidently, then, an eye whose fundus is distinctly 
visible through the aperture and becomes indistinct through 
the weakest convex lens, must emit parallel rays and be 
emmetropic. 



OPHTHALMOSCOPY. 79 

Xow suppose that the patient has a hyperopia eye in 
which rays of light are not refracted strongly; the rays 
reflected from the mirror into this eye evidently emerge 
divergent, and passing through the aperture tend to come 
to a focus behind the retina of the physician, but he exerts 
his accommodation enough to bring them to a focus on his 
retina and gets a distinct picture. If he now turns on a 
+ 1D. lens, it enables him to see distinctly without 
accommodating so much, and a -j- 2 D. enables him to 







Fig. 48. 

relax still further. When, however, he turns on a + 3, he 
can no longer see distinctly. Evidently the + 2 lens made 
the rays parallel and enabled him to see only by relaxing 
completely, while -f- 3 makes them convergent and brings 
them to a focus in front of his retina. Just as evidently 
it follows that the + 2 lens would enable the patient to 
bring parallel rays to a focus and hence is the measure of 
his hyperopia. 

Now suppose the patient to be a myope of 3 D., from 
whose eye convergent rays are given off. The rays from 
his eye passing through the aperture in the mirror are 
convergent and come to a focus in front of the physician's 
retina, forming no distinct image. A plus glass makes 
them only more convergent and less distinct. A — ID., 
however, makes details somewhat plainer, and with a — 3D. 



80 REFRACTION AND MOTILITY OF THE EYE. 

the rays have become parallel and he gets a clear picture of 
the fundus. By turning on stronger concave lenses he 
makes the rays divergent and still sees the fundus, but only 
by straining his accommodation. Evidently, a — 3D. lens 
made the rays parallel and is the measure of the patient's 
myopia. 




Fig. 49. 

In every one of these cases it is plain that the emergent 
rays were made parallel by the strongest plus or weakest 
minus glass with which the fundus can be seen plainly, 
since any different glass allows the observer to use his 
accommodation and so is a source of error. 

The macula is the portion of the retina on which rays 
must focus to secure distinct vision, and therefore it is 
theoretically the spot where we should estimate the refrac- 
tion; but the various light reflexes make the macula very 
difficult to see plainly and a bright light on it is so 
dazzling to the patient that in practice we examine a spot 
on the same plane, but between it and the disc, and take as 
our test of distinct vision the ability to see clearly the 
velvety appearance of the retinal surface or one of the 
minute blood vessels which run from the disc toward the 
macula. If we take as our object the large vessels, their 
calibre is so great that their surface is in a plane far 
anterior to the retina, and they are visible long before and 
after the smaller ones are blurred; while if we take the 



OPHTHALMOSCOPY. 81 

disc, it may have such a deep excavation that its surface 
is far behind the retinal plane. The observer, if not 
emmetropic himself, must be made so by glasses or must 
make a corresponding correction of his ophthalmoscopic 
finding. For instance, if he is a hyperope of 2 D. and sees 
the details of the fundus best with a + 4 D. lens, it is 
evident that 2 D. served to correct his own error and that 
the patient's hyperopia instead of 4 D. is only half that 
amount. If the observer is myopic 2 D. he must either wear 
a — 2 D. lens or add 2 D. to his estimate of his patient's 
error. 

While the beginner will find it very difficult to relax 
his accommodation, and for a long time will be obliged to 
allow a margin for error due to it, ophthalmoscopy is a 
fairly reliable method of estimating simple hyperopia and 
myopia, but when it comes to astigmatism, especially of low 
degree, the possibility of error becomes an actual probabil- 
ity. "We have seen how, for practical purposes, we can 
consider all the refracting media of the standard eye as 
equivalent to a single convex spherical lens which again 
may be considered as composed of two convex cylinders 
with their axes at right angles to each other. Now, if we 
draw on a sheet of white paper two lines equally black, 
crossing each other at right angles, and regard them 
through a strong convex cylinder, held at its focal distance, 
the line which corresponds to the axis of the c}4inder at 
once appears very much blacker and plainer than the other 
which is absolutely unchanged. Then, if the second 
cylinder is placed over the first with its axis at right angles, 
the other line corresponding to the axis of the second glass 
is brought out. Evidently each line is refracted through 
the cylinder whose axis corresponds to its direction. 

Now, suppose an eye whose media are equivalent to a 



82 



REFRACTION AND MOTILITY OF THE EYE. 



lens composed of two unequal cylinders, the one with its 
axis horizontal, or 180, emits parallel rays, while the 
vertical one, or 90, is weaker and so emits divergent rays. 
If near the macula of this eye we could find a vertical vessel 
crossing a horizontal at right angles, it is evident the 
vertical one depends on the cylinder whose axis is at 90 for 
its distinctness, while the other is seen by the one at 180. 




Fig. 50. 



When the physician looks through the aperture of his 
ophthalmoscope, he is able to see the horizontal line, since 
rays from it emerging parallel focus exactly on his retina. 
Rays from the vertical line, emerging somewhat divergent, 
come to a focus behind his retina, and the line is not so 
distinct ; and if he accommodates to see the latter he at once 
loses the former, since they focus on different planes. If he 
turns a + 1 D., the horizontal line (focussing in front of 
his retina) is blurred, so that plainly in the axis of 180 
the eye is emmetropic. The vertical line is visible with a 
+ 2 D. and blurs with anything stronger, so it is plain that 



OPHTHALMOSCOPY. 83 

the patient is hyperopic 2 D. in the axis 90 and that if he 
wears in front of his eye a convex cylinder of a 2 D. with 
the axis at 90, both meridians will be equal in their refrac- 
tion and both emmetropic. 

Evidently the process has been to find the strongest 
plus or weakest minus glass with which each of the two 
vessels can be seen, and the difference between them repre- 
sents the amount of astigmatism. Unfortunately, we can 
seldom find two vessels crossing near the macula and we 
are obliged to take for our horizontal one of the fine vessels 
running from the disc toward the macula, while for the 
vertical we can view the extreme outer edge of the disc 
itself or some small vertical vessel on some other part of 
the field. Xeither is the astigmatism always vertical or 
horizontal and the procedure practically is about as 
follows : — 

If, in looking through the aperture, the fundus is 
plainly visible, but anything stronger than a + 2 causes 
the vessels running at the axis 45 to blur, while vessels at 
right angles or 135 *are blurred by any lens stronger than 
-f- 4 ; evidently this patient can be made emmetropic by 
use of a -f~ 2 cylinder axis 45 combined with a + 4 axis 
135, which is equivalent to + 2 sph. + 2 axis 135. 

In compound myopic astigmatism nothing can be dis- 
tinctly seen through the aperture, but as minus lenses are 
turned on, one meridian clears before the other. Suppose 
that with a — 2 the vessels running at an axis of 75 appear 
distinct, while those at 165 are only clear when — 4 has 
been reached ; evidently emmetropia can be produced only 
by combining — • 2 ax. 75 with — 4 ax. 165 or — 2 sph. — 
2 ax. 165. Or one meridian may be hyperopic, while the 
one at right angles is myopic. 

By aid of the ophthalmoscope we can appreciate differ- 



84 REFRACTION AND MOTILITY OF THE EYE. 

ences of level in the fundus and measure them with con- 
siderable accuracy. For instance, if the refraction of the 
retina is emmetropic, while a small vessel on the disc can 
be seen with a + 3 D., it is evident that it must be nearer 
the cornea, and it has been estimated that a difference of 
level of one-third of a millimetre causes a refractive differ- 
ence of about 1 D. Similarly we can measure by the 
refraction the depth of the excavation or the height of an 
exudation, and determine from time to time whether it is 
increasing or not. 

The Indieect Method. — Unfortunately there are 
many eyes which cannot be successfully examined by the 
direct method. For instance, in cases of high astigmatism, 
while each meridian can be measured separately, it is often 
impossible to get any view of the fundus as a whole, owing 
to the great distortion. Many times it is possible to get 
a good idea of the fundus by the indirect method, though it 
has too many possibilities of error to be of value in 
estimating the refraction. This method requires much 
practice before any facility is acquired and is very com- 
monly neglected in America, but will amply repay effort 
enough to master it. 

The observer, when throwing the light of his ophthal- 
moscope into a highly myopic eye to detect opacities in the 
media, has probably been greatly surprised to see distinctly 
vessels or the optic nerve. The explanation is this : — 

Just as the rays of light from a candle, after passing 
a convex lens, form an inverted image in front of the lens 
which may be caught on a screen (see Fig. 21), so 
the rays from the highly myopic eye, emerging convergent, 
form an inverted image in the air in front of the eye, 
which the observer looking through the aperture of his 
ophthalmoscope can see clearly. By the aid of a strong 



OPHTHALMOSCOPY. 



85 



convex lens we can make any eye myopic and so get an 
inverted aerial image in front of the auxiliary lens. 

The lamp is arranged as for the examination with the 
erect image, and the observer, sitting at a short arm's length 
from the patient, holds with his thumb and forefinger a 
large convex lens of about 15 D. in front of the patient's 
eye. The results are best when the lens is distant from 
the pupil about its focal length, or about two and a half 




Fig. 51 



inches, and it must be held perfectly still. Both indica- 
tions are met by resting the little finger on the patient's 
forehead and at the same time, if necessary, the patient's 
upper lid can be slightly raised by the ring finger without 
moving the position of the lens. The patient, if under 
atropin, should be directed to look at the student's fore- 
head as this brings the disc and macula directly in view. 
When not under atropin, he should look past one's ear into 
the distance, lest he contract his pupil in accommodating. 
The light from the mirror is then thrown through the lens 
into the eye, and on emerging forms an aerial image of 
the fundus between the observer and the lens, which he can 
see by moving his head backward and forward in the path 



86 



REFRACTION AND MOTILITY OF THE EYE. 



of light till it comes in view. This image is within a few 
inches of the eye and therefore seen only by accommodating. 
Hence it is* much easier to see it by turning on a -f 3 or 
+ 4 lens, or, if one is sitting near enough to require it, 
a +.7. 




Fig. 52. 



This method shows a much larger portion of the 
fundus at a time than the direct, and consequently each 
portion of the field is magnified less. The stronger the 
auxiliary lens, the more extensive the field, while it is also 
somewhat dependent on the refraction of the eye, being 
larger than normal in n^opia and smaller in hyperopia. 

While the direct method enlarges the image from 
12-16-20 diameters, according to the refraction present, the 
indirect with a 15 D. lens magnifies from 2-4-8 diameters. 



OPHTHALMOSCOPY. 87 

It will be noticed at once that in moving the auxiliary 
lens from side to side, the fundus seems to move in the 
same direction, so that to get a view of the temporal side 
the lens is moved slightly toward the temple, or to look 
below the disc, the lens is moved slightly down. 

By showing more of the fundus at one time and being 
less disturbed by errors of refraction, the indirect method 
may give a much better general idea of the conditions pres- 
ent than the direct, and should be much more generally 
employed than it is. 

The refraction can also be measured by the indirect 
method, but as accuracy depends on the absolute absence 
of accommodation on the part of the physician, the method 
has fallen into disuse. 

Inequalities in the eye ground are determined by mov- 
ing the convex lens in front of the eye from side to side 
without losing sight of the disc. If the nerve be excavated, 
the edge will move in front of the floor in the direction in 
which the lens is moving. If the head of the nerve 
projects, its summit will move backward and forward with 
the movements of the lens. 

The lenses which are furnished with the ophthalmo- 
scope for use in this method are generally too small to be 
of much use, since the larger the lens the more emergent 
rays it collects and the more extensive the view. Also the 
presence of scratches on the lens is very objectionable, not 
so much because they obstruct the light as because they 
attract the observer's eye, and he insensibly looks at them 
and overlooks the aerial image between him and the lens. 
Consequently a large lens, protected by a very wide rim of 
hard rubber or metal, makes a most satisfactory lens, which 
can be laid down without the glass coming in contact with 
anv surface. 



CHAPTER IV. 
RETINOSCOPY. 

There has been considerable dispute as to the terms 
"Retinoscopy" and "Skiascopy," some preferring one and 
some the other on etymological grounds, but the two are 
used as equivalent terms and refer to the estimation of the 
refraction of the eye from a careful study of the shape and 
movements of a beam of light thrown from a mirror and 
reflected from the retina (retinoscopy) or of the so-called 
shadow which borders the reflex (skiascopy). When 
thoroughly mastered, it is by far the most generally reliable 
and accurate of the objective tests, since it measures not 
the refraction of one medium but of all, requires the 
simplest apparatus, and, moreover, is equally useful in 
young children, in the stupid and illiterate, the nystagmic 
and amblyopic. 

The one essential to the exact estimation of the refrac- 
tion of any eye with transparent media is the absence of 
its accommodation, which can be secured by the use of a 
cycloplegie like atropin or homatropin. 

Eetinoscopy should be done in an absolutely dark 
room, the light from a lamp being reflected into the eye to 
be examined, from a plane mirror, while the movements of 
the reflex are studied by the physician through a central 
aperture in the mirror. An electric retinoscope on the 
same principle as the electric ophthalmoscope, but with a 
plane mirror, may be used. The position of the light may 
be adjusted according to the preference of the observer, 
some having it placed on the level of and slightly behind the 
eye to be examined, while others place it a few inches 
over the patient's head. My preference is for the last, but 
the two chief requisites are that it shall be so placed that the 
light may be reflected from the mirror into the eye with as 
(88) 



RETINOSCOPY. 89 

little tilting of the mirror as possible, and that no light 
shall reach the eve except that from the mirror, since the 
reflex or illumination of the retina will be much more 
distinct against a dark background. The closer the light 
is to the mirror the greater the intensity of illumination, 
but aside from this the distance of the light from the 




Fig. 53. 

mirror is of no great importance, the only important dis- 
tance being that between the eyes of the observer and 
observed. Neither is special apparatus for modifying or 
condensing the light necessary, the only requisites being that 
the light be bright enough to illuminate the retina and yet 
not so bright as to dazzle the patient. This can be secured 
with an ordinary candle, if necessary, or regulated as 
desired by the ordinary Argand gas burner or frosted 
incandescent bulb. If one always places the light on the 



90 



REFRACTION AND MOTILITY OF THE EYE. 



side and very close, an asbestos chimney is advantageous 
to modify the heat, and some of them are made with 
an iris diaphragm to modify the light if desired. These 
have a certain advantage in that they do not light up the 
whole dark room as does the uncovered light. 

The choice of a mirror is much more important. 
While the work was originally done with a concave mirror, 




Fig. 54. 

it is now the custom to use a plane one, and for fear of 
confusion the theory of the plane mirror only will be 
considered. 

The essentials of a good mirror are the glass, in which 
all are about equally good, and the aperture, in which many 
are defective. The aperture is made to see through, and if 
it consists simply of a small 'area in which the silver is 
scraped from the mirror, it collects dirt and moisture, 
which are difficult to remove because of the metal backing, 






RETIXOSCOPY. 91 

and are a constant source of indistinctness and annoyance. 
Therefore the hole should be drilled smoothly clear through 
the glass. Moreover half the mirrors sold have apertures 
entirely too small for practical use. One can see much 
more distinctly through the larger apertures. The refrac- 
tion of the physician is of no importance, except that he 
must be able to see distinctly, either with or without glasses, 
the movements of the reflex. If he has a refractive error 
for which he cannot compensate, and especially if he is 
presbyopic, he should wear a glass which gives him sharp 
vision at the chosen distance. This may advantageously be 
fastened to the back of the retinoscope. Many a physician 
who has abandoned retinoscopy in disgust has done so 
because of an imperfect instrument or defective vision. 

If we place a convex lens at its focal distance from a 
screen and with a mirror throw a beam of light so that it 
falls on one edge of the lens, and move the mirror .so that 
the beam of light moves across the lens to the other edge, 
we shall see if we look behind the lens that the illumination 
moves in the same direction as the rotation of the mirror. 
Exactly the same thing occurs if we move the lens nearer 
or farther from the screen, except that being out of focus 
the illuminated area is larger and dimmer than before. 
Evidently, then, it makes no difference in the actual motion 
of the reflex whether the focus be in front or behind the 
screen ; whether the eye be emmetropic, myopic or hyper- 
opic. But if we watch the illuminated area through the 
lens, there is a decided difference, as the reflex moves with 
the mirror when the lens is within its focal length of the 
screen, and against when the lens is further away. 
Evidently the path of the ingoing rays may be disregarded 
while the emergent ones are the ones to be studied. 

It is assumed that the patient's accommodation has 



92 REFRACTION AND MOTILITY OF THE EYE. 

been abolished by a cycloplegic and that the room is 
absolutely dark except for the light over the patient's head. 
In examining the right eye, .the physician should be seated 
at a distance of one metre, should preferably place the 
mirror in front of his own right eye, and vice versa. In 
order that the rays emerging from the patient's eye may 
be as nearly macular as possible, he should be directed to 
look at the physician's forehead or ear or at some distant 
object beyond the edge of the mirror. 

Now let us study for a moment the path of the rays of 
light and the movements of the reflex in the emmetropic 
eye. 




^ O 

Fig. 55. 

Rays of light thrown from the mirror into an emme- 
tropic eye come to almost an exact focus on the retina and, 
being reflected, emerge perfectly parallel in the form of 
a cylinder whose diameter corresponds to the size of the 
pupil. The reflex in such a case, coming from a small, 
brightly illuminated area on the retina, is particularly 
bright and well defined. Now if the top of the mirror be 
tilted forward, so that the focus of light travels slightly 
downward on the patient's retina, evidently the direction of 
the emerging cylinder of rays will be directed upward, but 
as long as any of the peripheral rays reach the eye of the 
observer, the reflex is visible and appears to have moved 
lower down in the patient's eye. If the bottom of the 
mirror be in turn tilted forward, the focus of light travels 



RETINOSCOPY. 93 

upward on the retina, the cylinder of emergent rays is 
directed downward, and as long as it is visible the reflex 
appears to have moved upward. 

The same thing holds true of lateral tilting of the 
mirror. But since the emergent cylinder of rays has a 
very small diameter, it is evident that a very slight dis- 
placement of the reflex will turn the cylinder so far that 
none of the rays will reach the observer's eye at all, and the 
reflex will at once disappear. In the examination of the 
emmetropic eye, therefore, the reflex is very bright and dis- 
tinct, appears to move with the mirror and moves very fast, 
a very slight tilting of the mirror causing it to disappear. 
Evidently, if the pupil be thoroughly dilated, the cylinder 
of rays is larger and remains visible an appreciably longer 
time. 



Fig. 56. 

In the hyperopic eye the rays of light from the mirror 
tend to come to a focus behind the retina, consequently the 
illumination is not concentrated on a point, but forms 
diffusion circles on the retina depending on the amount of 
hyperopia. Consequently, the reflex is not as bright and 
distinct as in emmetropia. The rays of light emerging 
divergent, form a cone of light, whose base is toward the 
observer, and as only the central rays reach him, the reflex 
seems much fainter than it really is. If the top of the 
mirror be tilted forward, the illuminated spot on the 



94 REFRACTION AND MOTILITY OF THE EYE. 

patient's retina moves down, the cone of rays is turned 
upward, and it appears to the observer that the reflex has 
moved down with the mirror. If the bottom of the mirror 
be tilted forward, the illuminated area on the retina moves 
upward, the emergent cone is directed downward and the 
reflex appears to have moved upward with the mirror. 
The same holds true of lateral tilting of the mirror in either 
direction. If the rays of light be very divergent, it is 
evident that the mirror can be tilted considerably before the 
direction of the emergent cone of rays has been changed so 
that none of them reach the observer. Consequently, in 
hyperopia the reflex moves with the mirror in every 
meridian, and in proportion to the amount of hyperopia it 
is dimmer and appears to move more and more slowly. 



Fig. 57. 

u ■ - 

In high myopia the rays of light reflected from the 
mirror tend to focus in front of the patient's retina, and on 
the retina itself only diffusion circles form. The rays 
reflected from the retina emerge convergent in the form of 
a cone whose apex is toward the observer and crossing at 
the apex come to him divergent, so that only a portion of 
them reaches his eye. Consequently the reflex seems dim 
in proportion to the amount of myopia. At the apex of the 
cone, where the rays of light cross, an aerial image is formed 
of the illuminated area on the retina, which the observer 
sees instead of the area itself. Now if the top of the 
mirror be tilted forward, the illumination on the retina 



RETINOSCOPY. 95 

travels downward and the emergent cone of rays is directed 
upward, but to the observer the aerial image at the apex of 
the cone appears to have moved upward, or against his 
mirror. If the bottom of the mirror be tilted forward, the 
illuminated area on the retina actually moves upward, the 
emergent cone is directed downward, and the aerial image 
appears to have moved downward. The same is true of the 
lateral movements, and if the myopia is high, the aerial 
image can move through a considerable arc before dis- 
appearing from view. In myopia, then, the apparent 
motion of the reflex is opposite or against that of the 
mirror, while the reflex is dim and the motion slow in pro- 
portion to the amount of myopia. 




Fig. 58. 



Xow let us consider a very low myopia of one dioptre. 
Rays of light from the mirror do not form very large 
diffusion rings and therefore the reflex is comparatively 
bright. The emergent rays are slightly convergent, forming 
a slender cone of light whose apex is exactly forty inches 
from the nodal point of the eye. If, now, the observer 
stations himself at, say, a distance of twenty inches from 
the eye, or inside the crossing point of the rays, and tilts 
the top of his mirror forward, the illuminated area moves 
downward, the emergent cone is directed upward, and the 
observer, since he is within the crossing point of the rays, 
sees the reflex itself, which therefore appears to move 
exactly as in hyperopia and emmetropia. If he gradually 
increases his distance from the patient's eye, the motion 



96 REFRACTION AND MOTILITY OF THE EYE. 

continues to be with his mirror as long as he can see the 
reflex light itself, but the instant he gets beyond the point 
at forty inches, where the convergent rays cross, he ceases 
to see the real reflex and sees instead the aerial image, 
while the reflex appears at once to move against his mirror. 
The point where the rays cross is spoken of as the point of 
reversal, and if we can measure exactly the distance of this 
point from the eye, we can estimate exactly the lens required 
to make the emergent rays parallel. . For instance, it 
requires a 1 dioptre lens to bring parallel rays to a focus at 
one metre, therefore an eye whose point of reversal is 
exactly one metre away converges the emergent rays just 
one dioptre too much. In other words, it is myopic one 
dioptre, and a concave glass of 1 D. will make the rays 
exactly parallel. If the point of reversal be established at 
one-half metre, the amount of myopia is two dioptres; if 
at ten inches, four dioptres; while, if the point of reversal 
was found at eighty inches, or two metres, evidently the 
myopia is only a half dioptre. 




Fig. 59. 

If the patient is emmetropic or hyperopic, the emerg- 
ing rays of light form no natural point of reversal and we 
have to establish one by interposing a convex lens of known 
strength. If he be emmetropic, a convex lens of 1 D. will 
cause the emerging parallel rays to form a point of reversal 
at exactly one metre, a+2D. will establish the point of 
reversal at a half metre, and so on. Conversely, rays of 
light which after the interposition of a + 1 D. form a point 
of reversal at forty inches, must have been exactly parallel 



RETINOSCOPY. 97 

beforehand, and the eye must have been emmetropic. One 
metre is the distance usually chosen for retinoscopy as a 
matter of convenience, and the whole retinoscopy consists 
in finding the lens which, when placed before the examined 
eye, will cause a crossing of the rays just in front of the 
nodal point of the observer's eye. If, on tilting the mirror, 
the reflex appears rather indistinct and sluggish, and moves 
with the mirror, we are certain we have to do with a very 
hyperopic eye whose rays are divergent. We place in the 
trial frame before the patient's eye gradually increasing 
convex lenses till we find the weakest one which will cause 
the reflex to move against the mirror. If this is accom- 
plished by a -\r 5, we remember that one dioptre of this was 
used in bringing the rays to a focus after they had become 
parallel, consequently the remaining 4 D. were just enough 
to make them parallel, and evidently the eye had a hyper- 
opia of 4 dioptres. 

Suppose, in another case, the reflex is very bright and 
moves very rapidly with the mirror, indicating probable 
hyperopia of very slight degree. By placing convex lenses 
of gradually increasing strength before the eye, we find 
when we reach -J- 1.25 that the motion first becomes against 
the mirror and we decide that, after deducting one dioptre 
for the distance at which we are working, the remaining 
quarter of a dioptre was all that was required to make the 
emerging rays parallel. Suppose again that the reflex 
moves with the mirror, but is reversed by a + .50 D. : 
evidently the rays must have been convergent a half 
dioptre, since, if they had been parallel, it would have 
required a + 1 D. to reverse the motion. 

Suppose the motion is distinctly against the mirror, 
indicating the convergent rays of myopia, and we find that 
by placing before the eye a — 3D. lens, we get a "with the 



98 REFRACTION AND MOTILITY OF THE EYE. 

mirror" motion at a metre. Evidently, if with a — 3D. 
the rays come to a focus at forty inches, it will require 
another — ID. to make them parallel, and therefore, a 
— 4 D. is the measure of the myopia present. When we have 
established a point of reversal at forty inches, we have 
carried the process too far in hyperopia, since a dioptre less 
would have made the rays exactly parallel ; consequently we 
deduct the extra dioptre. 

In myopia, on the other hand, where we have estab- 
lished the point of reversal at forty inches, we have not 
carried the process far enough, and as we know that it 
requires another dioptre to make the rays parallel, we add 
that extra dioptre. 

(The working distance of a metre is an arbitrary one, 
as we can select any distance we choose as long as we make 
the proper allowance. At thirty inches we should allow 
1.50 D., at twenty inches 2 D., etc.) 

Astigmatism. — If we think of the refracting media of 
the eye as equivalent to a single strong convex lens, which 
is again equivalent to two cylinders of equal strength, but 
with their axes at right angles, it follows that as long as 
the cylinders are equal the rays of light emerge in all 
meridians equally divergent or convergent or parallel, while 
the eye is either hyperopic, myopic or emmetropic and can 
be measured by the retinoscope. But if the imaginary 
cylinders are not exactly equal, the rays of light must be 
refracted more in one meridian than in the other and they 
no longer come to a focus at the same point; hence the 
name (a-stigma) (Fig. 50). 

According to the strength of these imaginary cylinders 
the rays pass out from an illuminated point on the retina 
in one meridian and divergent in the other (simple hyper- 
opia astigmatism), divergent in both, but more in one 



RETINOSCOPY. 99 

than in the other (compound hyperopia astigmatism) , 
parallel in one and convergent in the other (simple nryopic 
astigmatism), convergent in both, but more in one than in 
the other (compound myopic astigmatism), and lastly they 
may be divergent in one meridian and convergent in the 
other, in which case one meridian is hyperopic and the 
other myopic, and the astigmatism is called mixed. 

Simple Hyperopic Astigmatism. — Imagine an eye 
composed of two cylinders, one with its axis horizontal, or 
180°, of such strength that emergent rays refracted by it 
are parallel, while the vertical one at 90° is weaker and 
sends out divergent rays. 

If the light from the retinoscopic mirror be thrown 




o 



Fig. 60. 



from a distance of one metre into this eye, a small area on 
the retina is illuminated and the rays reflected from this 
pass out of the eye and return to the observer. The rays in 
the vertical plane have been refracted by the horizontal 
cylinder so that they emerge parallel; and since they all 
reach the observer, the reflex seems to reach clear across the 
pupil vertically, the horizontal rays refracted by the weaker 
vertical cylinder are divergent, and as only the central ones 
reach the observer, the reflex seems much smaller and 
narrower horizontally. Consequently, the reflex appears 
to the observer to be in the shape of a bright vertical band 
of light which is narrower, and with straighter edges the 
higher the astigmatism (and the band always is exactly in 
line with the axis of the correcting cylinder). In simple 



100 REFRACTION AND MOTILITY OF THE EYE. 

hyperopia we saw that the rays emerged from the eye in a 
cone whose apex was toward the eye. In hyperopic 
astigmatism they form a cone whose upper and lower 
surfaces are flattened till they are parallel, while the sides 
are divergent still. If the top of the mirror be tilted 
forward, the illuminated area on the retina moves down- 
ward and the cone is tilted upward, but as long as any of 
the undermost rays reach the observer's eye, the reflex is 
visible and appears to have moved with the mirror; but 
since the cone is very narrow vertically, a very slight motion 
of the mirror will displace the cone so much that it is no 
longer visible. Consequently, the vertical motions of the 
reflex are with the mirror, but are very rapid. When the 
mirror is tilted to the right, the illuminated area on the 
retina travels to the right, while the cone is tilted to the 
left, but remains visible as long as any of the lateral rays 
reach the observer, and as the lateral diameter of the cone 
is very great, it is visible for a long time. Hence from 
side to side the band moves very distinctly with the mirror 
and much more slowly than in the vertical meridian. If we 
place before the eye a + 1 D. sph. and examine the vertical 
motion again, the rays forming the top and bottom of the 
cone have become convergent so that they intersect just 
before reaching the observer's eye, and the vertical motion 
is against the mirror, while the lateral motion still remains 
with the mirror. Since it required exactly a 1 D. lens to 
establish this point of reversal at a metre, we know that the 
rays must have been parallel before. Now, disregarding 
entirely the vertical motion, we add stronger and stronger 
convex lenses till we find the lowest one which will reverse 
the lateral motion which, let us suppose, requires a + 4 D. 
We know that one dioptre of this was required to bring the 
rays to a focus after they were parallel, and the remaining 



RETIXOSCOPY. 101 

three must have been sufficient to make them exactly 
parallel. If we substitute a + 3 cylinder axis 90, so as to 
correct only the divergent rays, all the light reflected from 
the eye will emerge in parallel rays, and by the addition of 
a -f- 1 sph. the motion will be reversed in all meridians 
exactly as in emmetropia. 

In Compound Hypeeopic Astigmatism the rays of 
light emerge divergent in both meridians, but more so in 
one than in the other. 

They therefore form a somewhat flattened cone of rays, 
only the central ones reaching the observer's eye. In the 
meridian nearest the normal, fewer rays reach the eye than 
in emmetropia, so the reflex still has the appearance of a 
band which is at right angles to the more divergent rays and 
therefore indicates exactly the axis of the correcting glass; 
but the band is not as well defined as in simple astigmatism 
and in high grades is very difficult to see, but becomes more 
and more distinct as we get our meridian corrected. Since 
all the rays are divergent, the reflex moves with the mirror 
in all directions, but more slowly in the meridian of greatest 
divergence. Let us suppose that by adding stronger and 
stronger convex lenses a band of light gradually develops 
pointing toward the axis 45 on the trial frame, and that 
when we have reached a-f 3D. sph. the up-and-down 
motion is reversed ; it is evident that the meridian would be 
made emmetropic by a + 2 cyl. axis 135. Passing on to 
the lateral motion of the reflex, we find that reversed by a 
+ 5, and evidently this meridian is made emmetropic by 
a + 4 cyl. axis 45. 

The eye as a whole, then, would be made emmetropic 
by a combination of a + 2 ax. 135, with a -|- 4 ax. 45, which 
is equivalent to + 2 sph. 3 + 2 ax. 45. We should be 



102 REFRACTION AND MOTILITY OF THE RYE. 

able to prove this result by demonstrating a point of 
reversal at one metre with a + 1 D. in all meridians. 

We get the same result, if we consider the lens which 
corrects the least ametropic meridian as the measure of 
hyperopia while the difference between this and the glass 
which neutralizes the other meridian represents the astig- 
matism corrected by a cylinder with its axis corresponding 
to the band. 

Simple Myopic Astigmatism. — In this condition rays 
of light emerge parallel in one meridian and convergent in 
the one at right angles to it ; consequently, the rays form a 
flattened cone, just as in simple hyperopic astigmatism, 




Fig. 61. 

except that the apex instead of the base is directed toward 
the observer. If the rays forming the sides of the cone 
emerge parallel, the reflex will appear to reach clear across 
the pupil, while in the meridian at right angles, where the 
rays are either convergent or divergent according to whether 
the observer is within or without the point of reversal, the 
reflex is much narrower. Therefore, in myopic astigmatism 
the rule holds good that the band of light in the pupil lies 
in the axis of greatest ametropia and therefore indicates 
the axis of the correcting glass. In the case supposed then, 
there will be a bright reflex band running horizontally 
across the pupil. 

Lateral tilting of the mirror will cause the reflex to 
move with the mirror from side to side and the motion will 
be reversed by a + 1 D. sph. at forty inches. Vertical tilt- 



RETINOSCOPY. 103 

ing on the other hand will cause the reflex to move up and 
down against the mirror since the rays cross before reaching 
the eye and form an aerial image of the reflex. If they 
cross exactly at forty inches, the patient must be myopic 
exactly one dioptre in that meridian, since a lens of that 
strength would be required to make the rays parallel. If 
it requires a — 2 D. to reverse the motion, the astigmatism 
must be — 3, since, as in simple myopia, if — 2 brings the 
point of reversal to forty inches, it will require another 
dioptre to make the rays parallel. Evidently by placing 
before the eye a — 3D. cyl. ax. 180, the eye will be made 
emmetropic in all meridians, which can be proven as in the 
other cases. In astigmatism as in simple myopia, if the 
error be less than 1 D., the motion of the reflex will be with 
the mirror, but will be reversed by a weak convex lens, and 
the correction should be estimated exactly as in myopia, but 
in one meridian at a time. 

In Compound Myopic Astigmatism the rays of light 
are convergent in all meridians, but more so in one than in 
the other. In such a case the motion will be "against" in 
both, and each meridian must be estimated separately. If 
the motion of the reflex is reversed by — 2 in one meridian 
and by — 4 in the other, there is evidently 2 D. of astigma- 
tism, while to the myopia of — 2 sph. must be added a 
dioptre for the distance at which we are working, so that the 
eye would be made emmetropic by a — 3 3 — 2 cyl. 

The correction in mixed astigmatism is estimated in 
the same way. The light in one meridian passes from the 
eye in convergent rays, while that in the other is divergent; 
consequently the reflex moves with the mirror in one and 
against in the other. Each should be estimated separately, 
adding one dioptre to the minus glass and subtracting one 



104 REFRACTION AND MOTILITY OF THE EYE. 

from the plus. The correcting glass is estimated by com- 
bining the two. 

If the vertical reflex is with the mirror and reversed by 
a + 2, the refraction in that meridian is + 1 axis 180, and 
if the lateral motion is against the mirror and reversed by 
— 1 2, the refraction in that meridian would be ■ — 3 axis 90, 
and the two may be combined in either of three ways: 




Fig. 62. 



(+ 1 ax. 180 C — 3 ax. 90), (—3 sph. C -f 4 ax. 180), 
(+ 1C- 4 axis 90). 

Eetinoscopy is by far the most generally useful and 
accurate method of estimating the refraction of an eye, and 
with comparatively little practice the beginner can make an 
approximate estimate of the conditions present. But the 
determination of the exact point of reversal, the exact axis 
of astigmatism, in other words, the estimation of refraction 
to the fraction of a dioptre which is advisable in many 
cases, is a much more difficult matter. For this reason the 
student is strongly advised to purchase one of the artificial 



RETINOSCOPY. 



105 



e) T es on the market and practice assiduously (Fig. 62). 
By aid of one of these he can artificially create an eye with 
any variety of ametropia and study the movement of the 
reflex and the adjustment of suitable lenses. It is very 
probable that some persons have a special aptitude for the 
method, because of keen sight, and then there are many 
others who never become very accurate through a defective 
faculty of observation. 

In some cases the method is not applicable at all, 
especially when opacities in the cornea and lens so scatter 
the emergent rays that the motion of the reflex cannot be 
distinguished with certainty. 






Fig. 63. 



Spherical aberration is also a great puzzle to the 
beginner, especially when the pupil is dilated widely. In 
simple hyperopia, for instance, of low degree, the rays in the 
centre of the reflex come out divergent and cause a "with" 
motion of the reflex. But the peripheral rays are refracted 
more strongly, come out convergent and move against the 
mirror. The reflex in such a pupil appears in the form of 
a peripheral ring of light moving against the mirror with a 
central area moving with it. As stronger and stronger 
lenses are placed before the eye, the "with" reflex gets 
smaller and more rapid as the point . of reversal is 
approached, while the peripheral ring gets broader and more 
distinct in proportion. But the peripheral refraction is of 
no importance to the patient, because these rays are 



106 REFRACTION AND MOTILITY OF THE EYE. 

ordinarily cut off by the iris, and the centre of the reflex 
alone should be the guide. Evidently if the observer is 
not very careful and sharp-sighted, he will overlook the 
faint central "with" motion and underestimate the hyper- 
opia. In myopia the peripheral refraction is also greater 
than the central, and as concave lenses are placed before the 
eye, the motion of the centre of the reflex is reversed and 
becomes "with" sooner than the peripheral circular reflex. 
If one keeps adding concave lenses till he has reversed the 
more conspicuous reflex, he has evidently overestimated the 
amount of myopia, which is often a serious mistake. 

When the spherical aberration is negative, as in conical 
cornea, the central rays are refracted more strongly than 
the peripheral and the conditions are reversed. If the 
student will bear in mind that from the standpoint of dis- 
tinct vision the centre of the pupil is the important part, 
and be guided only by the motion of the centre of the reflex, 
he will soon cease to be troubled, and some day we may dis- 
cover a drug which will paralyze the ciliary muscle without 
the troublesome dilation of the pupil. 

This same spherical aberration is the source of great 
trouble in estimating astigmatism. At the axis, 180 for 
instance, we have seen that the round reflex becomes a 
horizontal band. The peripheral ring is also elongated in 
the same direction, very often to such an extent that only 
the top and bottom are visible. We therefore have three 
bands across the pupil, and, as the peripheral ones move 
more slowly, the central one appears to overtake first one 
and then the other as the mirror is moved. As the point of 
reversal is approached, the peripheral bands move "against" 
before the central one. 

If the horizontal plane of the eye is not exactly on 
the same plane as that of the observer, but is turned slightly 



RETINOSCOPY. 107 

up or down, one of the peripheral bands is not seen and 
there are left only two which approach each other and 
recede like the blades of a pair of scissors. Theoretically, 
the same phenomena should be observed in astigmatism 
with the axis vertical, but practically it is not as noticeable, 
probably because — as we shall see in another chapter — we 
have much more practice in discriminating vertical lines 
than horizontal, from the prevailing shapes of our alphabet. 
Some authorities ascribe this scissors reflex to a tipping of 
the crystalline lens slightly forward or backward in its 
fossa. The scissors reflex clinically, therefore, almost al- 
ways means a hyperopic astigmatism axis horizontal, and to 
estimate it correctly one must disregard the peripheral rays 
entirely and study only the central ones. Here, as in simple 
hyperopia and myopia, the danger is of underestimating the 
first and overestimating the last. 

Retinoscopy Without a Cycloplegic. — Since retinos- 
copy depends entirely for its accuracy on a complete relaxa- 
tion of the ciliary muscle, it might be supposed that the 
method was useless except under cycloplegia. To a certain 
extent this is true, but after some practice the method will 
be found very valuable in many cases which are not under 
atropin. As we have already seen, there is a large class of 
patients in which a cycloplegic is not desirable, either be- 
cause it is inconvenient or because, since they come for 
improvement of vision and not for relief of strain, a full 
correction is neither necessary nor even desirable. 

When under atropin. the patient is directed to look at 
the observer's forehead or ear during the process of retinos- 
copy. If he is not under atropin, he will at this distance be 
accommodating a dioptre, and if he is hyperopic, very much 
more, but if he be directed to gaze past the observer's ear 
into the obscurity of the dark room which is particularly 



108 REFRACTION AND MOTILITY OF THE EYE. 

arranged so that there shall be nothing to fix his attention, 
his accommodation will be in a state of physiological relax- 
ation which is often almost as great as in cycloplegia, and 
with the manifest advantage that the moderately contracted 
pupil shuts out the peripheral rays which are so trouble- 
some. 

The working distance should be one metre, and if the 
observer does not carry the correction quite to the point of 
reversal, but only finds the glass with which the original 
motion of the reflex is no longer perceptible, this will very 
closely approximate the actual refraction without adding 
1 D. in myopia or subtracting 1 D. in hyperopia. Any 
motion "with" the mirror certainly means hyperopia more 
or less, while any motion "against" the mirror only probably 
means myopia. 

By examination of a long series of patients by both 
methods, the writer has convinced himself of the useful- 
ness and accuracy of the method when used in connection 
with other tests. It is the commonly accepted theory of 
accommodation, whether invariably true or not, that the 
ciliary muscle contracts and expands equally in all 
meridians and that the lens is, in health, equally elastic. in 
all meridians. Under these conditions, even if the relax- 
ation be not complete, the error will be the same in all 
meridians, and so the difference between meridians which 
represents the amount of astigmatism will be a constant 
one and the same as under cycloplegia. Thus, as we shall 
see, astigmatism is generally due to optical defects in the 
cornea rather than the lens, which we can estimate with 
approximate accuracy by another method, and the two 
serve as checks on each other. The tendency in retinoscopy 
without cycloplegia is certainly to overestimate myopia, but 
it becomes less and less as proficiency increases with prac- 



RETINOSCOPY. 



109 



tice. It certainly is not a scientifically exact method of 
estimating the total refraction, nor is it dependable in very 
young and unintelligent patients, but as a rapid method of 
determining errors of refraction at the first visit it is very 
useful, and in the very large class of patients who simply 



Fig. 64. 



want glasses to improve vision and not to relieve strain and 
who would certainly be very much disgusted with a full 
correction, it is, in conjunction with other tests, entirely 
sufficient. 

In retinoscopy it is the custom to place a trial frame 
on the patient and use appropriate lenses from the trial 
case in estimating the refraction. This has the advantage 



HO REFRACTION AND MOTILITY OF THE EYE. 

of permitting a very exact measurement of the error owing 
to the many fractions of a dioptre contained in the case. 

In estimating astigmatism some surgeons make use of 
cylinders, but it is not usual, since it is just as easy to use 
spherical lenses, neutralizing the meridians of greatest and 
least curvature separately. For clinical convenience the 
writer devised a series of lenses set in hard rubber frames 
which are small enough to be carried in the vest pocket, and 
which contain graduations enough to meet the requirements 
of ordinary clinical work, though of course they would not 
suffice where extreme accuracy was desirable. They can be 
held by the patient before the eye, but it is much easier and 
more rapid for the physician to hold the frame against the 
patient's eyebrow himself. Of course, this reduces the 
working distance between the lens and mirror materially, 
for which due allowance must be made. In the ordinary 
case this is about thirty inches, calling for a correction of a 
dioptre and a half, instead of the customary one dioptre. 
He has found it a very great convenience, not only in clin- 
ical work, but in the preliminary examination of private 
patients, where it is often desirable to know approximately 
the amount of error present in persons who have called for 
some other difficulty. 



CHAPTER V. 

THE PUPIL— CYCLOPLEGICS— MIOTICS— STATIC 
REFRACTION. 

We have alluded very briefly to the physical effects 
which occur in the iris and lens during the change of focus 
from the far point to the near, but have made no explana- 
tion of the nervous mechanism by which they are con- 
trolled; indeed, the changes themselves are not by any 
means simple or well understood. We have seen that the 
iris is largely composed of blood vessels, that it has a 
sphincter, by stimulation of which the pupil is narrowed, 
and that it contains elastic tissue which contracts when the 
sphincter relaxes, and dilates the pupil, not widely but 
moderately, till a balance is obtained between the tonicity 
of the sphincter and the elastic fibres. The contraction of 
the sphincter takes place under the influence of a branch of 
the third nerve, running from the anterior part of the floor 
of the fourth ventricle very close to the allied third nerve 
nuclei, for accommodation and convergence. This con- 
traction takes place when the retina is exposed to light in 
proportion to the sensitiveness of the retina and the 
brightness of the light. The stimulation reaches the optic 
tract through the optic nerves and is carried to the oculo- 
motor nucleus by a special set of communicating fibres, 
and because of the decussation of optic nerve fibres both 
pupils respond to stimulation of either retina. Pupillary 
contraction also occurs automatically with convergence and 
accommodation, all this being governed by closely asso- 
ciated nuclei of the same nerve. It also occurs when the 
aqueous is deficient, which explains why the pupil cannot be 
dilated with an open wound in the anterior chamber. It 
also occurs upon the administration of certain drugs such 

cm) 



112 REFRACTION AND MOTILITY OF THE EYE. 

as eserin, morphin, in epileptic attacks, etc., and in certain 
nervous diseases. Conversely, dilation of the pupil occurs 
when the contractile stimulus is diminished by darkness. 
When the eye is focussed for distant objects with corre- 
sponding relaxation of accommodation and convergence, the 
pupil also relaxes and is semi-dilated. This also occurs 
when its aqueous humor is abnormally abundant with a 
consequent increase in tension, as in glaucoma, during 
fatigue, during the administration of certain drugs, in 
certain nervous diseases and as the result of emotions. The 
impulses from the cervical sympathetic seem to play an 
important part in the dilation of the pupil, not only by 
inhibiting the action of the sphincter, but also by a specific 
dilating action of their own. Entirely aside from the 
action of the elastic fibres in the iris, the pupil seems then 
to be under the influence of two antagonistic forces : that of 
the third nerve which, under appropriate stimulation 
contracts the pupil and when not acting permits moderate 
dilation, and that of the cervical sympathetic which, when 
stimulated, not only inhibits the sphincter action, but 
causes a wide dilation such as occurs under the influence of 
fear, anger, etc. Section of the sympathetic in turn not 
only removes the dilating influence, but removes the only 
restraint on the action of the sphincter and hence results in 
a marked contraction of the pupils. According to this 
theory the excision of the sympathetic ganglia in the neck 
has been practiced with the idea of narrowing the pupil in 
glaucoma. 

The effect of many drugs seems to be wholly or in 
part independent of these nerve influences. For instance, 
a tropin causes a wide dilation of the pupil and eserin a 
corresponding contraction, but both are said to produce 
their full effect after the division of all the nerves and 



THE PUPIL. 113 

even after extirpation of the eyeball. They appear to act 
directly on the muscular tissue of the sphincter or on its 
nerve endings, and hence their action is entirely local. 
Other drugs apparently affect the pupil partly or wholly 
by their action on the blood vessels of the iris. For 
instance, cocaine causes an iritic ischemia and tends to 
dilate the pupil by stimulating the sympathetic, and though 
it has no cycloplegic action of itself, is often added to 
cycloplegics to accomplish this very purpose. Dionin, on 
the other hand, partly, perhaps, by its lymphagogue 
reduction of tension, but chiefly by a temporary edema of 
the iris, will sometimes contract a pupil that has been 
paralyzed by atropin. 

As the result of the crossing of the fibres of the optic 
nerves so that filaments from each eye reach both oculo- 
motor nuclei, both pupils in health react together to 
stimulation of either retina, and therefore should be prac- 
tically equal. Any inequality (anisocoria) is pathological 
and must be due not to perceptive anesthesia, but to some 
interference with the innervation of the sphincter. It may 
be caused by conditions in the iris itself, such as adhesions 
between the margin of the pupil and the lens, which prevent 
entirelv or in part either contraction or dilation, or by 
traumatism by which the sphincter is temporarily par- 
alyzed, and this paralysis frequently involves only a portion 
of the fibres so that the pupil is oval or irregular. It may 
be caused by an increase in the tension of one eye, as in 
simple glaucoma, as well as the frequent accidental poison- 
ing by infinitesimal doses of atropin, and this should be 
constantly in the clinician's mind. It is often caused by 
paralysis of the third nerve, occasionally alone, but 
generally associated with paralysis of accommodation and 
some of the extrinsic muscles, and may be a valuable guide 

8 



114 REFRACTION AND MOTILITY OF THE EYE. 

to the localization of cerebral lesions. An anisocoria may 
also be caused by the sympathetic nerves, either an undue 
irritation on one side causing unilateral dilation, or paral- 
ysis causing contraction. 

The size of the pupils varies widely even in health, 
being much greater in youth, in conditions of anemia and 
malnutrition and in refractive anomalies, through which 
there is present a certain amount of retinal anesthesia, and 
the sphincter of the iris often partakes- in the exhaustion of 
the ciliary muscle. In older patients smaller pupils are 
the rule and in the very aged the blood vessels become 
hardened, the iris inelastic and an actual miosis (extreme 
contraction) is present. Scales are provided for measur- 
ing and recording the diameter of the pupils in millimetres, 
when this is desirable. 

The reaction to light should always be tested, the best 
method being that described in the chapter on ophthalmos- 
copy, the direct method giving us a view of the pupil 
magnified by a 7 D. lens, while by arranging the light 
behind the plane of the patient's eye we can change the 
illumination instantly from practical darkness to that of 
the condensed light from a concave mirror. For the con- 
sensual reaction it is necessary to use daylight or have the 
light so arranged that both pupils are visible without being 
strongly illuminated, where the light can be condensed on 
one with a lens, while the other is carefully watched. 

The reaction to light is generally a very good and very 
delicate test of the perception of light, but it is not to be 
forgotten that there are a number of conditions in which 
the reaction may fail though the perception is normal. 
This would occur in the conditions already alluded to, but 
unless the other eye were . also affected, the consensual 
reaction would answer the same purpose. For instance, in 



CYCLOPLEGICS. 115 

locomotor ataxia the reflex arc connecting the optic nerve 
with the nucleus of the third nerve is interrupted on both 
sides. As a result neither pupil contracts to light stimula- 
tion of the retina, but the third nerve itself being 
unaffected, the pupil reacts as usual during accommodation 
and convergence. This is the Argyle-Kobertson pupil. In 
these nervous conditions a very marked contraction of the 
pupil known as spinal miosis is often present. Conversely 
there are patients whose reacting pupils show that the eyes 
see, but who are brain blind, as for instance in some cere- 
bral hemorrhages, uraemias and hysterias. 

A cycloplegic is a drug which will, for the time being, 
paralyze the ciliary muscle and prevent the use of the 
accommodation, while a mydriatic is one which has the 
power of dilating the pupil and may or may not have any 
effect on the ciliary muscle. The cycloplegics all have a 
mydriatic action, but not all mydriatics are cycloplegics. 
The two words are very commonly, though incorrectly, used 
as though both implied paralysis of accommodation. 

Atropin and hematropin are the drugs most commonly 
employed, but there are a number of others less popular, 
but useful. The following table shows the duration of the 
paralysis from the different drugs which will be noticed to 
van- widely: — 





Complete 


Complete 




Paralysis 


Recovery 


Atropin 


4 days 


15 days 


Homatropin 


12 hours 


2 days 


Scopolamin 


12 hours 


6 days 


Hyoscyamin 


3 days 


8 days 


Duboisin 


2 days 


8 days 



If a solution of appropriate strength is dropped into 
the eye, it is in the course of a few minutes absorbed by 
the conjunctival vessels and penetrates the anterior cham- 



116 REFRACTION AND MOTILITY OF THE EYE. 

ber so that it acts directly on the nerves of the ciliary 
body and the iris, causing paralysis first of the latter, as 
shown by the dilated pupil, and soon afterward of the other, 
as shown by a growing inability to see distinctly close at 
hand. Unless very carelessly used, there should be no 
systemic effect from these instillations, but sometimes the 
solutions, if used too freely, are carried through the 
lachrymal passages to the nose and throat and so produce 
by absorption a dry throat, flushed face and a rapid pulse, 
in fact, all the effects of a mild belladonna poisoning. To 
prevent this possibility, it is well to have the patient make 
pressure over the punctum. at the inner corner of the eye 
for a couple of minutes with his finger or a pledget of 
cotton. Occasionally patients have an idiosyncrasy against 
these drugs which cause a severe conjunctivitis of the 
follicular type. The tendency to conjunctivitis and derma- 
titis can be much lessened by combining with the cycloplegic 
an astringent, like zinc sulphate, in the strength of one 
grain to the ounce. Solutions of atropin in oil, like castor 
or olive, are often much more effective than aqueous ones. 
Oily solutions must be made with the alkaloid and not the 
salts of atropin which are not soluble. 

Patients should always be warned of the expected effect 
of the drug on vision, since otherwise they are often very 
much alarmed at the inability to see things near at hand. 
Cycloplegics should never be used when there is the least 
suspicion of a tendency to glaucoma, since there are many 
cases on record where acute attacks have been precipitated 
by their use, and the stronger ones are to be avoided in the 
case of nursing women, lest they accidentally absorb enough 
to influence the mammary secretion. 

Indications for Cycloplegics. — Whether the use of 
cycloplegics should be a routine one or not, has been — and 



CYCLOPLEGICS. 117 

probably always will be^— a subject of dispute among 
ophthalmologists, one school claiming that no scientific 
refraction is possible without, and the other that their use 
is rarely necessary. So bitter has been the controversy 
and so positive have been the assertions on the subject that 
the whole profession to-day has a vague impression that, on 
the one hand, an ophthalmologist who does not regularly 
use a cycloplegic has not done his full duty by his patient, 
and on the other hand, that if an examination under a 
cycloplegic does not relieve the symptoms the eyes cannot 
be at fault. 

A moment's thought will convince one that while there 
is, without doubt, a foundation for both beliefs, neither one 
is unreservedly true. The one advantage of a cycloplegic 
is that it paralyzes the accommodation, and it has asso- 
ciated with it the obvious disadvantages of dilating the 
pupil widely and so calling into use the peripheral portions 
of the refracting media which nature evidently did not 
intend should be used for this purpose. Its use is always a 
source of discomfort and often of alarm to patients and has 
contributed as much as anything to the vogue of the 
refracting optician who is not permitted to use drugs and 
widely urges that they are never necessary. 

To me it seems that the use of a cycloplegic is an 
individual question depending on two personal equations : 
those of the physician and his patient. The physician who 
has fallen into the habit of always using a cycloplegic, 
depending on his retinoscopic tests without the careful use 
of the ophthalmoscope, ophthalmometer and other auxiliary 
-• -. is utterly lost when it comes to the examination of an 
eye when the pupil is small and the accommodation active, 
and thinks everyone else must be equally at sea, while as a 
matter of fact, an experienced man using care and judg- 



118 REFRACTION AND MOTILITY OF THE EYE. 

ment will, in suitable cases, reach the same result either 
with or without a cycloplegic. In elderly people, whose 
accommodation is no longer active, a cycloplegic is seldom 
necessary. On the other hand, in the very young, the 
ignorant, the nervous, and especially in those cases where 
there is a spasm of accommodation, no very trustworthy 
result can be obtained without a cycloplegic. The purpose 
of the examination also is very important in arriving at a 
decision. There is a very large class of patients who come 
with no discoverable symptoms except poor vision due to 
presbyopia, hyperopia, astigmatism, etc. In these it is a 
matter of minor importance whether the entire error be 
corrected, as long as a partial correction gives the desired 
keenness of vision. Indeed it commonly happens that a 
full correction under atropin is very much less satisfactory 
to these patients than a partial one. In myopia and astig- 
matism, where there is much greater danger of over-correct- 
ing the error and so producing accommodation strain, 
greater care is necessary and much more frequently the use 
of a cycloplegic, but in young, vigorous patients the over- 
correction is not necessarily harmful. 

There is another large and increasing class of patients 
whose vision may be perfectly good, who consult the oculist 
because of headache, chorea, double vision, or who come 
to have the eyes, as possible causes of various reflexes, 
excluded. In such cases it is desirable to be absolutely 
accurate in the estimation of refraction, and a cycloplegic 
is certainly called for. Between these two extremes come a 
series of cases where the employment or non-employment of 
a cycloplegic is a matter of individual judgment and skill. 

The choice of a cycloplegic is also a matter of impor- 
tance, since it is better to use none at all than to imagine 
the accommodation is paralyzed when it is not. Atropin 



CYCLOPLEGICS. 119 

is the oldest and perhaps the most efficient of our drugs, 
and yet occasionally we see cases in which even prolonged 
use does not cause complete relaxation, while with homa- 
tropin and the weaker C} r cloplegics, I am convinced that 
many oculists are in the habit of estimating the refraction 
of patients, who, while they have a widely dilated pupil, are 
but partially under the influence of the drug. 

In the selection of a cycloplegic the physician must be 
guided by the patient's age, occupation, the nature of his 
difficulty and the kind of refractive error present. For 
practical purposes all the cycloplegics may be divided into 
two classes: one, of which atropin stands at the head, in 
which the cycloplegia is complete and lasts many days, and 
the other represented by homatropin in which the drug is 
not as powerful and its effect evanescent. 

An elderly person in whom the accommodation has 
become inactive, if indeed any cycloplegic is necessary, 
could use homatropin, while in a child with a very active 
accommodation the same drug would be entirely untrust- 
worthy. A busy man cannot afford to use atropin which 
will certainly keep him from his work at least a week, while 
he might be able to use homatropin at the week's end with 
the certainty that he would be able to use his eyes by the 
beginning of another week. A patient whose tired eyes 
are giving rise to reflex symptoms should be examined 
under atropin, since there is more likelihood of discovering 
his total error, and also because he will actually benefit 
from the long rest. 

The same may be said of myopic eyes in which there 
is a tendency to spasm of the accommodation which only 
yields to a strong cycloplegic, and in which there is very 
often chorioidal change which is benefited by the enforced 
rest. It is fortunate that in high myopia the cycloplegics 



120 REFRACTION AND MOTILITY OF THE EYE. 

cause much less disturbance of vision, since the myopic 
pupil is large normally, and the patient is not dazzled by 
the increase in its size and can still see things near at hand, 
because he is myopic. 

Commonly, when ordering a cycloplegic, the patient 
should be directed to wear temporarily a smoked glass to 
avoid the unpleasant dazzling. 

Atropin should be used in a one-per-cent. solution, one 
drop being instilled into each eye three times a day. 
These drugs can be obtained in the form of gelatin 
ophthalmic discs of corresponding dose, which are allowed 
to dissolve in the eyes. 

I have expressed a decided distrust of homatropin, as 
it is commonly used, but if used freely enough, a satisfac- 
tory cycloplegia may be obtained in suitable cases. It 
should be prescribed in very small quantity, as it is very 
expensive, the patient being directed to instill a drop of 
two-per-cent solution several times a day during the day 
prior to examination. If used in the office a drop is 
instilled every five minutes for half an hour, or one of the 
gelatin discs allowed to dissolve in the eye, but it is impor- 
tant to remember that it requires at least an hour for the 
full cycloplegia to develop. 

This method sometimes causes a blood-shot eye, and 
while it dilates the pupil, the completeness of cycloplegia 
is open to doubt, since it has been demonstrated that in 
some cases only a partial paralysis occurs. One portion of 
the ciliary muscle is at rest while another is still active or 
even in a state of cramp, thus causing the appearance of a 
pseudo-astigmatism which is not normally present. 

Cocaine is very commonly combined with homatropin. 
both in the solutions and in the ophthalmic discs, and ii 
certainly appears to increase the effect, but cocaine has ne 



CYCLOPLEGICS. 121 

paralyzing action on the ciliary muscle and probably only 
increases the dilation of the pupil which is a positive dis- 
advantage, while it also, in many cases, causes temporary 
changes in the epithelium of the cornea which appears hazy 
and embarrasses the physician in his retinoscopy and the 
patient at the test cards. Another disadvantage of homa- 
tropin is this, that in patients who for various reasons 
should wear the entire correction of their error, the effect 
of the drug passes off before they have even got their glasses, 
and hence, when they are put on, the patient cannot see 
through them, while atropin passes off so slowly that the 
effect of the drug is insensibly replaced by that of the 
glass. 

On the whole my advice to beginners, at least, is to 
learn to get along without a cycloplegic as much as possible, 
by methods which will be described, but when one is neces- 
sary, select one which shall be thorough and have it used 
long enough before the examination so that the result shall 
be final and conclusive. The weaker drugs should be 
reserved for patients who cannot be satisfactorily examined 
without a cycloplegic and yet cannot spend the time 
necessary for one of the slower and stronger remedies. 

When cycloplegia is complete the patient is entirely 
without focussing power. With a suitable convex lens for 
instance, he can still read fine print or see the hairs in the 
hair-optometer, but they are distinct only at a fixed distance 
and become blurred when brought either nearer or further 
from the eyes. 

We have reason to suppose that the accommodation is 
not completely at rest when there is a notable difference 
between subjective and objective tests. For instance, a 
patient who can still read his newspaper cannot be under 
the influence, unless he be naturally myopic. A patient 



122 REFRACTION Ai\D MOTILITY OF THE EYE. 

who, by retinoscopy, has shown a hyperopia of 3 D. and yet 
by the test cards reads 20/20 with a+lD. must be using 
his accommodation. A patient who makes no expression 
of preference between a -j- 1, -j~ 2 or -f- 3, the vision being 
equally clear with all, must be accommodating. A patient 
whose objective myopia is — 2 and yet who prefers — 5 
in the subjective tests, must be accommodating. On the 
other hand, a patient whose dark room test shows myopia, 
while at the test cards he reads 20/20 without glasses, was 
evidently accommodating during retinoscopy. In other 
words, a patient ought to obtain the best vision with a 
glass approximately equal to the dark room correction, and 
any discrepancy throws suspicion on the accuracy of the 
examination or the completeness of the cycloplegia. 

Static Refraction. — We have already seen that in the 
emmetropic eye, parallel rays of light come to a focus on 
the retina without any effort on the part of the patient. 

In the short or hyperopic eye they tend to focus behind 
the retina and clear vision is only secured by contraction of 
the ciliary muscle. In the myopic or long eyeball, the rays 
focus in front of the retina, and clear vision is only secured 
by bringing the object of regard nearer, till the rays do 
impinge on the retina. In astigmatism clear vision is 
secured, if at all, by one or the other of these methods, or 
by a combination of the two. All varieties of ametropia, 
therefore, mean either a reduction of distant vision or the 
imposition of abnormal muscular effort in seeing distinctly. 

The static refraction of an eye means the estimation of 
the glass or combination of glasses which shall enable it to 
focus parallel rays on its retina when the accommodation 
is completely relaxed by a cycloplegia 

In the estimation of this total error the subjective or 
trial case method is to be considered as the court of last 



STATIC REFRACTION. 123 

resort. In young children, idiots, and illiterates, we are 
compelled to rely on other tests, and with skill can make a 
very close approximation to the truth, but with an intelli- 
gent patient the subjective method is decidedly more 
delicate. 

Of the objective methods, the ophthalmoscope is 
hardly to be considered a refractive instrument except of 
the roughest sort. The ophthalmometer, as we shall see, 
estimates only the astigmatism of the cornea and is not 
claimed to be accurate within a half dioptre. The retinos- 
cope furnishes our most reliable objective test, but with it 
we study the course of rays which emanate from a com- 
paratively large region about the macula, while distinct 
vision depends on the macula alone, whose minute bundle 
of rays may have a distinct refraction of their own. The 
patient is the only one who can tell with the utmost delicacy 
whether rays focus exactly on his macula, because this 
gives him the clearest possible vision, and he can very often 
distinguish differences of an eighth of a dioptre or of two 
or three degrees in the axis of a cylinder. It must be 
remembered, too, that the static refraction does not mean 
the strongest plus or weakest minus combination which 
gives the patient normal vision, but the combination of 
any nature that gives him the greatest possible visual 
acuity, since his accommodation is paralyzed, and it is not 
necessary to make any allowance for strain. 

Before using a cycloplegic it is advisible to make a 
routine examination of the eye which should give an 
approximate idea of the refraction as well as reveal any 
disease present. At the same time the distant vision both 
with and without glasses should be noted. In estimating 
this vision we not only measure the refraction but 
determine the delicacy of the retina, regardless of muscular 



124 REFRACTION AND MOTILITY OF THE EYE. 

strain, so that we may have some standard of vision to be 
attained or surpassed after the induction of cycloplegia. 

One of the most essential requisites of good refraction 
is a good trial frame, one strong and rigid enough to remain 
squarely and firmly on the nose and in which the lenses 
can be so adjusted that their optical centres are opposite the 
middle of the pupil, perpendicular to the visual line and at 
the same distance from the eye at which it is intended the 
correcting lenses shall be finally worn, viz., just as close to 
the cornea as they can be without being swept by the eye- 
lashes. We now seat the patient before the test card and, 
examining each eye separately, note his vision without 
glasses and thus, guided somewhat by our retinoscopic 
examination, place spherical lenses in the frame in grad- 
ually increasing strength so long as the vision continues to 
improve. When this point has been reached we resort to 
cylinders, beginning with weak ones and, if they are of 
advantage, increasing till the maximum vision is reached. 
In determining the axis of the cylinder, it is best to accept 
not the one indicated by the retinoscope or ophthalmometer, 
but that which gives the clearest vision, turning it a few 
degrees in either direction to be certain that the patient 
always prefers one definite axis. If he seems uncertain, 
the cylinder should be rotated first one way and then the 
other, noting carefully the exact points where the patient is 
conscious of indistinctness. The point half way between 
may be assumed as the meridian of sharpest vision. To be 
sure that one has the combination giving the maximum 
acuity it is best to make experiment with spheres and cylin- 
ders, both slightly stronger and weaker, but the final selec- 
tion ought to be one which is blurred by the superposition 
of the weakest lens of whatever sort. This can be con- 
veniently done by placing over the correction a so-called 



STATIC REFRACTION. 125 

"cross cylinder," a weak plus cylinder ground at right 
angles to a weak minus one, thus at the same time increas- 
ing one meridian and diminishing the other one. In these 
tests it is best to be guided not so much by what the patient 
says as by the accuracy with which he reads the letters on 
the trial cards which should be changed from time to time 
as he becomes familiar with them. 

The determination of the static refraction does not 
mean that we necessarily prescribe the full correction for 
the patient to wear. The final prescription will depend on 
various factors, such as the age and occupation of the 
patient, the symptoms from which he is suffering and the 
amount and character of his error. 

When patients complain simply of defective vision it is 
very seldom necessary to subject them to cycloplegia, since 
the object can be as well, and often better, accomplished 
without it. In myopia and myopic astigmatism we 
frequently use cycloplegics to avoid over-correction and 
consequent strain. In these conditions it is customary to 
give the full atropin correction at once unless the error is 
an extreme one, in which the glass might possibly cause 
discomfort if worn for the first time. 

In patients with reflex symptoms, such as headache, 
we correct all or nearly all the astigmatism and as much of 
the hyperopia as, in our judgment, the patient can stand 
without discomfort. If he is young he has a stronger 
accommodation and will be restive under a correction 
which in middle life is easily born. If the symptoms are 
more serious, as in migraine, strabismus and the like, or the 
case is one in which the object is to exclude the eyes as a 
factor in some reflex condition, it is advisable to order the 
full correction for constant wear, and if necessary keep up 



126 REFRACTION AND MOTILITY OF THE EYE. 

a moderate cycloplegia till the patient's accommodation 
becomes relaxed enough to permit it. 

Miotics. — Eserin and pilocarpin have an action op- 
posite to that of the cycloplegics in that they place both the 
sphincter of the iris and the ciliary muscle in a state of 
tonic contraction. Consequently the pupil becomes very 
small and the eye is in a constant condition of accommoda- 
tion. When strong solutions are used, eserin often produces 
a feeling of tension in the eye with headache and sometimes 
nausea. The action of the miotics is of shorter duration 
than that of the cycloplegics and is less powerful, and their 
application in diseases of the eye does not fall within the 
scope of this chapter. In weak solution, however, eserin 
has a powerful tonic effect on the ciliary muscle. In many 
cases of lack of tone in accommodation after illness or 
overwork the use of a solution of eserin (gr. Yso-^sa, one 
drop t. i. d.) will markedly increase the comfort of the 
eyes and restore the ability to work. Naturally, however, 
the ciliary muscle should not be compelled by any such 
means to a continuous compensation for refractive errors. 



CHAPTEB VI. 
HYPEROPIA. 

Hyperopia is the refractive condition of an eye in 
which parallel rays of light tend to come to a focus behind 
the retina and therefore form on the retina a diffusion 
circle of light instead of a point. Evidently the only rays 
which can come to an exact focus on the retina are those 
which were more or less convergent before entering, and 
since the emergent rays would take the same path, the rays 
emerging from a hyperopic eye are more or less divergent. 
Under these conditions, in a state of rest, the hyperope can 
form no distinct, sharp image of any object either far or 
near, but by using his accommodation enough he can see 
with the greatest distinctness at a distance, hence the 
hyperope is called "farsighted" by the laity. 

Evidently there may be either or both of two conditions 
present as the cause of hyperopia: either the refracting 
media of the eye have less than normal action on rays of 
light so that the focal point is too far back, or the retinal 
screen may be too near the refracting surfaces. The first 
condition is spoken of as refractive hyperopia and may be 
due to a cornea which is natter than normal, either from 
accident of birth or disease or to a diminished refractivity 
on the part of the lens. This tends to take place regularly 
with advancing years as the lens becomes less bulky and 
natter in its curves. Extreme instances of refractive 
hyperopia occur in eyes in which the lens is wanting, as 
after a cataract operation or dislocation of the lens. 

When the hyperopia is due to the shortness of the 
eyeball, it is called axial hyperopia, and this is much the 

(127) 



128 REFRACTION A^D MOTILITY OF THE EYE. 

commoner form. Most animals have axial hyperopia and 
that was probably the condition of human beings before the 
advance of civilization made the use of the eyes for near 
objects so much more important. Certainly the great 
majority of infants and young children are hyperopic, since 
before attaining full size their eyeballs are shorter in every 
diameter than later in life. As they reach maturity, the 
axial hyperopia tends to grow less and less as the eyeball 
increases in its antero-posterior diameter, while in old age, 
as we have seen, the hyperopia tends to increase again by 
the flattening of the lens, this time being of the refractive 
type. Of course, an eye in which the retina has been 
pushed forward by a tumor would be hyperopic, but 
generally speaking, hyperopia may be said to be a con- 
genital difficulty which the great majority of individuals 
never entirely outgrow. Such eyes are optically imperfect, 
but are, as a rule, perfectly healthy in contradistinction to 
the myopic eye, which is almost always a more or less 
diseased eye. 

The vision in hyperopia varies, with the age of the 
individual and the amount of his error. Distant vision is 
possible only by exercise of more or less accommodation, 
but as long as this is active, the resulting vision is remark- 
ably good. Being in constant use for distance as well as 
near vision, the ciliary muscle of the hyperope becom.es 
regularly very much larger than in emmetropia, nor does 
it relax as completely. If we place before a hyperopic eye 
a convex glass which shall converge the rays of light so as 
to make accommodation unnecessary, the eye will see 
clearly till a glass is reached which converges the rays too 
much, when vision becomes indistinct. 

The hyperopia indicated by the strength of the 
strongest glass which permits clear vision is spoken of as 



HYPEROPIA. 129 

manifest. If, however, we place the eye under the influence 
of a cycloplegic, a much stronger glass will be accepted, 
showing that previously the patient had been unable to 
completely relax his accommodation. This extra hyperopia, 
which is revealed by a cycloplegic only, is the concealed or 
latent hyperopia, while the manifest and latent together 
form the total hyperopia. The manifest hyperopia varies 
greatly from day to day in some individuals, according to 
their ability to relax the ciliary muscle at the time. In 
very young people there is commonly a very slight ability 
to relax and therefore the manifest hyperopia is very small 
and the latent correspondingly large. Indeed it frequently 
happens that the contraction of the ciliary muscle is so 
great that the eye will accept no convex glass at all, in 
which case the entire amount is latent or concealed. With 
advancing years more and more of the error becomes 
manifest and less of it latent, so that in old age the 
hyperope will accept without a cycloplegic a glass which 
corrects his entire error. As long as he is able by the aid 
of accommodation to have distinct distant vision, his 
hyperopia is spoken of as facultative, while, when he can- 
not overcome his error by straining his accommodation, 
his distant vision falls below par and his hyperopia becomes 
absolute. Since the accommodation regularly fails with 
advancing years, even the slightest errors become in time 
absolute. 

If the hyperope can see distinctly at a distance only by 
using his accommodation, he is at even a greater disadvan- 
tage when it comes to near vision. The emmetrope in doing 
work at a distance of ten inches is obliged to accommodate 
4 D., but the hyperope who has to accommodate 2 D. to see 
distant objects like his rival, and then 4 D. more to read at 
ten inches, is evidently accommodating G D. If he has 



130 REFRACTION AND MOTILITY OF THE EYE. 

this amount at his disposal, he can see as distinctly as the 
other, but at the expense of greater muscular effort. There 
comes, too, a time when the accommodation of the emme- 
trope is no longer sufficient for his needs and he becomes 
the victim of presbyopia or old sight. Evidently the 
hyperope who regularly needs greater accommodative 
power, will feel this defect first and suffer from a premature 
presbyopia in proportion to the amount of his error. 

The near point of the hyperope, then, begins to recede 
earlier than in emmetropia and finally even in the lower 
grades gets beyond infinity, after which distant vision also 
is below par. 

There is another set of symptoms which often causes 
the hyperope far more trouble than mere indistinct- 
ness of vision. We have seen that by accommodating he 
can make* both distant and near vision satisfactory for a 
long time, but accommodation is as distinctly a muscular 
effort as weight lifting. If the hyperope has a good ciliary 
muscle, leads a healthy outdoor life and seldom uses his 
eyes for close work, he may finally drift into a condition of 
absolute hyperopia without ever being conscious of fatigue, 
but if his error is high or his ciliary muscle a poor one, if 
he live a sedentary life and especially if he compels his 
eyes to do a great amount of continuous close work, as in 
school, he gradually develops a train of symptoms that 
eventually cause him great distress. First of all, perhaps, 
in the afternoon or evening after an unusually hard day's 
work, the accommodation suddenly becomes unequal to the 
strain, relaxes and, as he expresses it, "things get black 
before his eyes." After a few moments' rest he is able to 
go on with his accustomed clearness. Gradually these 
attacks begin to come earlier and earlier in the day, to last 
longer and come oftener, while the necessary periods of 



HYPEROPIA. 131 

rest are longer and more frequent. Finally there is a kind 
of a haze due to poor focussing, over every near object he 
looks at. Associated with this is pain which may be slight, 
but is very often of the severest type; pain in the eyes, 
frontal headache, which often lasts for hours after the 
overwork has ceased and sometimes becomes so continuous 
and severe as to lead to the suspicion of organic disease. 
At the same time lesser troubles appear, such as watering 
of the eyes, inflammation of the lids, conjunctivitis of a 
mild grade, drowsiness on attempting to do any close work. 
Unless the cause of the discomfort is discovered it is laid to 
the light, ventilation, organic disease, nervous tempera- 
ment, uric acid, malaria, or whichever of the terms for 
disguising ignorance is at the time popular. The patient 
drags out his more or less miserable existence till actual 
inability to see objects directs attention to the eyes, and 
when he gets his glasses, his symptoms magically disappear. 
In school children and young adults who cannot change 
their occupation, a habit of shirking tasks and giving 
inadequate attention is instinctively developed. Uncor- 
rected refractive errors account for much inattention and 
apparent stupidity in children. 

Hyperopia manifests itself frequently in the form of 
reflex disturbances in other parts of the body, such as 
indigestion with its allied evils, neuralgia, various neuroses, 
like neurasthenia, choreiform movements, cardiac palpita- 
tion and the like, and may be a contributing if not a 
primary cause of migraine and epilepsy. 

It is particularly to be noted that very many of the 
patients afflicted with these symptoms, especially when 
young, retain their keenness of sight at a distance and can 
for a moment or two read the finest of print close at hand. 
The beginner should guard against the error of assuming 



132 REFRACTION AND MOTILITY OF THE EYE. 

that because a young patient has a vision of 20/20/ he 
cannot have any refractive error. Indeed the presumption 
is in the other direction, since momentarily vision is a 
trifle sharper when the ciliary muscle is slightly contracted 
and a vision of 20/15 or 20/10 almost certainly indicates 
an accommodative effort which, if the patient were emme- 
tropic, would reduce his vision materially. 

As we shall discover in another chapter, there is a 
normal relationship between accommodation and conver- 
gence, and since the hyperope is constantly called on for 
an excess of the one, he very frequently also manifests an 
excess of the other, thus eventually developing either a 
latent or a fixed squint. Thus he has to make a choice of 
two evils. If he sees distinctly by accommodating, he 
immediately sees things double by over-convergence, while 
if he would avoid the diplopia, he must be content in many 
cases with less distinctness of vision. Sometimes he learns 
to accommodate without converging. Sometimes he is 
content with seeing poorly and occasionally he learns to 
converge his eyes as much as he pleases, while he avoids 
diplopia by suppressing one image. He therefore sees with 
one eye and squints with the other. Particularly is this the 
case when one eye is more hyperopic than the other, in 
which case vision both near and far is best in the eye with 
the lowest error, and consequently he fixes objects with this 
and turns the other in. 

Diagnosis. — Hyperopia may be suspected from a 
history of the symptoms detailed above. In the old from 
the failure of distant vision which was once very clear; in 
the young from the history of gradually increasing difficulty 
from continued use of the eyes, blurring of vision, watering 
of the eyes and especially from pains in the eyes and head- 
ache, which are relieved completely or in part by rest, and 



HYPEROPIA. 133 

aggravated by work. A positive diagnosis, however, can 
be made only from a careful objective examination which, 
withal, takes very much less time than the subjective tests. 
Especially to be relied on are the ophthalmoscope and the 
retinoscope, the uses of which in this condition have already 
been explained. 

Treatment. — The treatment of hyperopia consists 
possibly of complete rest for the eyes, when they have 
become irritable and painful, and certainly in the prescrip- 
tion of convex glasses which shall correct all or part of the 
error. It is in the strength of the correction ordered that 
the judgment is called into play, since many competent 
men simply correct the manifest error, while others, fully as 
competent, advise correction of practically the whole 
amount. To me it seems that each case must be studied 
by itself and the correction graded according to the con- 
ditions present, and here again we must distinguish 
between the patient who comes because his vision is failing 
and the one who complains of headache or reflex trouble. 

Patients of the first class are generally somewhat 
advanced in years, and if we order a glass which corrects 
the manifest hyperopia, the vision is brought up to normal 
and every wish is satisfied. If, however, we try to correct 
the total error of such a patient, we give him a glass 
stronger than he can readily use without a cycloplegic, and 
as regards vision, he is even worse off than before. After 
a time the patient's distant vision again falls below par, 
as his latent hyperopia becomes more and more manifest, 
and it is necessary to again correct it, till finally, in course 
of time, the entire error has been corrected. In younger 
persons who complain of tired eyes and mild headaches, I 
am accustomed to pursue the same course except that I am 
much more careful to reach the extreme limit of the 



134 REFRACTION AND MOTILITY OF THE EYE. 

manifest error. In cases in which there is headache of the 
severer and more continuous type, reflex symptoms, such as 
choreiform motions, tendency of the eyes to squint, and 
especially if there is reason to suspect spasm of the accom- 
modation, I invariably use a cycloplegic. In young 
children, where one has to depend entirely on retinoscopy 
and ophthalmoscopy, a cycloplegic — preferably atropin — is 
necessary, because the accommodation is normally very 
active and beyond control and one can place no dependence 
on the subjective tests like the test cards and clock dials. 

There is a certain routine procedure that each man 
must determine for himself, not only to save time, but to 
avoid forgetting details which ought to be recorded. For 
instance, a patient of 35 years comes complaining that in 
the afternoon his eyes get tired and water and feel as 
though they had sand in them and that he is unable to 
read at night because he gets drowsy or has a headache, 
while his vision for distance is very good. One suspects 
that such a patient is using his ciliary muscle too much 
and that it is becoming unequal to the strain, which would 
not be the case at this age, if he were emmetropic. We 
first test his distant vision at twenty feet on the trial cards, 
with each eye separately, and find V. 20/20 in each. 
Evidently he cannot be myopic, or his distant vision would 
be reduced. He may be emmetropic, but he may also have 
a hyperopia which he is compensating for by his accom- 
modation, or he may be astigmatic. To experiment at this 
point with trial lenses is waste of time, because it would be 
done at random ; so we at once take the patient to the dark 
room, place the light over his head and examine him with 
the retinoscope. We have him look clear across the dark 
room, being careful that there is nothing here which can 
particularly engage his attention and so stimulate his 



HYPEROPIA. 135 

accommodation. In this way his ciliary muscle has a com- 
plete physiological relaxation. The surgeon places himself 
at a distance of about a metre in such a position that his 
right eye is just outside the visual line of the patient who 
is looking off into the distance. When the light is reflected 
into the patient's eye, the reflex is very clear and distinct, 
unless the pupil happens to be very small, and with practice 
this will become much less embarrassing. Eetinoscopy 
with a small pupil is much easier if the room be absolutely 
dark and is almost impossible in a simple twilight. A 
"with the mirror" motion of the reflex, when the patient is 
not under atropin, invariably means hyperopia in that 
meridian. When we place before his eye a + 3 lens, the 
motion in all meridians stops, while with a + 3.50 it 
becomes "against" the mirror. It will be found that the 
strongest convex lens which just stops the motion, without 
deducting anything for distance, will represent very 
accurately the hyperopia. 1 

Eight here it is interesting to have the patient look at 
the physician's forehead or his ear, as he might do if under 
a cycloplegic; in doing this he accommodates and appears 
much less hyperopic than he really is. The whole secret of 
successful retinoscopy without a cycloplegic is to secure a 
physiological relaxation of the ciliary muscle by gazing off 
into the distance. Having measured the right eye, the 
physician moves his seat or has the patient change the 
direction of his gaze, so that the physician's left eye is just 
outside the visual line of the patient's left eye. Let us 
suppose that the motion in this eye is brought to a stop by a 

1 If this patient were placed under atropin, a point of re- 
versal at a meter would be developed with a + 4D, and deducting 
1 D. for distance, the result would be in the great majority of 
suitable cases almost the same as by the above empirical method. 



136 REFRACTION AND MOTILITY OF THE EYE. 

+ 3 as in the. other. We are sure that the patient has a 
hyperopia of at least 3 D. without any astigmatism. 

Now we examine each eye with the ophthalmoscope, 
noting incidentally the pupillary reaction and whether the 
media are clear and the fundus normal. The strongest 
convex glass with which the finest retinal vessels can be 
seen is a + 3, which is therefore the measure of the hyper- 
opia and corresponds with the retinoscopic tests. 

With a reasonable amount of skill and experience the 
observer can judge from any discrepancy between the 
retinoscopic and ophthalmoscopic findings whether there 
has been a reasonably complete relaxation of accommoda- 
tion during the first test. There is probably no accom- 
modation, unless the image is first formed at the macula, 
and as the patient instinctively avoids focussing the bright 
light from the ophthalmoscope on his macula, there is 
usually a complete relaxation of the ciliary muscle during 
the ophthalmoscopic examination. 

\ It is my custom next to examine the surface of the 
cornea with the ophthalmometer of Javal. This step is 
not absolutely necessary, but as explained in another 
chapter, most of the astigmatism met with is corneal and if 
the ophthalmometer shows that the cornea has no astigma- 
tism or none exceeding a half dioptre, it simply furnishes a 
third indication that we have to deal with a simple 
hyperopia. 

We now seat the patient before the test cards, and 
covering up the left eye, try to find the strongest convex 
lens with which he can retain his normal vision of 20/20. 
We are certain that if our patient were under atropin, he 
would accept a + 3 D. lens, but without the atropin we 
cannot expect so much. We first place a -f ID. sph. in 
front of the eye and find that the 20/20 line is still distinct 



HYPEROPIA. 137 

and immediately place over this a + 1*50, which reduces 
the vision several lines. When, however, we withdraw the 
1 D. from the combination, the vision immediately becomes 
normal again. The idea in placing the second lens before 
the eye before withdrawing the first is to prevent the 
patient using his accommodation while the lenses are being 
changed. When we try the same maneuver with a + 1-75, 
he fails to read the line correctly and evidently + 1.50 is 
the manifest hyperopia in this eye. 

We now cover the right eye and examine the left in 
exactly the same way, finding exactly the same result. All 
our tests have indicated that the eyes are exactly alike in 
their refraction and should have the same glass, but w^e find 
that by using both eyes together, a still further relaxation 
of the ciliary muscle takes place and that with a + 2 D. 
before each eye the binocular vision is still 20/20 and we 
prescribe this glass with the full confidence that it will 
relieve the symptoms complained of, and that without so 
diminishing the distant vision as to be disagreeable to the 
patient. It may be objected that the glass does not correct 
the entire error and that in near work it still leaves too 
much, work on the accommodation, but it must be remem- 
bered that the hyperope has an unusually powerful ciliary 
muscle which, by the aid of the glass, is perfectly able to 
do this work. This patient can wear eyeglasses if he 
prefers them, the precaution being taken that the optical 
centres of the lenses come directly opposite the pupillary 
centres. He is instructed to wear his glasses constantly 
for near and far work till, after an interval of several weeks, 
mptoms having abated, he is allowed to discard them 
on the street if he prefers, retaining them regularly for his 
near work. 



138 REFRACTION AND MOTILITY OF THE EYE. 

The so-called "fogging" method which amounts to the 
same thing, consists in putting over the eye a convex lens 
strong enough to blur the distant vision badly, and then 
adding to it gradually increasing minus lenses till the pa- 
tient's normal acuity is reached. This may be approxi- 
mately estimated before hand by having him read through 
the pin hole disc which excludes from the eye all except the 
practically unrefracted axial rays. 

Now let us consider another case, that of a boy of 12, 
who is brought because he is near-sighted, seeing badly on 
the street and holding his book close to his eyes in his 
efforts to read. On testing his distant vision, we find 
it 20/70 which inclines us to believe that the parental 
diagnosis may be right in spite of the rarity of myopia in 
children. Without wasting time in efforts to improve his 
vision at this time, we hurry him to the dark room, where 
the retinoscope shows a very faint reflex moving slowly but 
distinctly "with" the mirror. The defect is then not 
myopia, but a very high hyperopia, which we estimate as 
before by having the patient look into the distance while 
we find the lens which just stops the motion, a + 7 D. in 
this case. The ophthalmoscope shows the same amount in 
each eye, while the ophthalmometer shows no appreciable 
astigmatism. This patient has a hyperopia so high that 
one would expect at first sight all sorts of reflex pains and 
aches from which he has apparently been notably free. As 
a matter of fact, by straining his accommodation to the 
extreme, he has never had satisfactory distant vision and 
therefore he long ago ceased to try to see distinctly, while 
his case is still worse near at hand. Realizing uncon- 
sciously the futility of accommodating, he has acquired the 
habit of holding things close to his eyes so that his retinal 
images may be as large as possible, though indistinct. 



HYPEROPIA. 139 

When we place him before the trial cards, we find that 
a + 3 before each eye separately, gives him the maximum 
vision of 20/30, while with both eyes together he will 
accept a + 3. 75. As far as his distant vision is concerned, 
this would be a very satisfactory glass, but it would leave 
almost too much ciliary strain in distinct near vision. It 
must be remembered that this is a school child who is 
backward and will have to do an unusual amount of near 
work, that even with this glass he would have to accom- 
modate 3.25 dioptres more than his emmetropic neighbor, 
and while at his age his accommodative power is at its 
height, he has not used his accommodation like the ordinary 
hyperope, since it did no apparent good, and therefore his 
ciliary muscle is comparatively feeble. To prescribe this 
weak glass would improve his vision and stimulate him to 
force his eyes to work with which they are evidently not 
able to cope except for short intervals, and we should very 
shortly have the train of reflex headaches and symptoms 
from which he has hitherto been free. If we prescribe a 
+ 5 for him, we reduce his distant vision to 20/50, but 
inasmuch a-s he has never in his life before seen better than 
20/70, he will be encouraged to wear his glass constantly, 
while his vision will constantly improve as he gets used to 
the lenses till it again approximates the normal. At the 
same time we have placed his near vision well within his 
powers for years to come. 

Now let us consider the case of a child of 4, who is 
brought because of a beginning squint. We can test his 
vision by marbles or other small objects and find it good in 
both eyes, but when we repair to the dark room, we cannot 
use the retinoscope because he insists on looking on the 
mirror instead of across the room. The ophthalmoscope 
shows a hyperopia of a 4 D., but we cannot say that there is 



140 REFRACTION AND MOTILITY OF THE EYE. 

no astigmatism. We might estimate his error from the 
ophthalmoscope alone and be approximately correct, but the 
only exact way is to put him under the influence of a 
cycloplegic and estimate his error by the retinoscope, which 
is comparatively easy as he is interested in the process and 
watches intently. Since his eye is under atropin we make 
the customary allowance for distance of one dioptre in using 
the retinoscope. By this means we find his error to be 
exactly 4 D. with no astigmatism. We prescribe nearly the 
full correction for its effect on the squint, though we are 
certain that as soon as he comes out of the influence of the 
atropin he will see somewhat better without his glasses. 
We can, by continuing the atropin a long time, get him to 
see well with his full correction, gradually discontinuing it 
as the glass is accepted. 

Let us suppose the case of a school girl of 15, who 
comes complaining that she cannot see things on the street, 
while she constantly suffers from headaches, is getting 
nervous and irritable, and has to hold her book very close to 
her eyes in studying. A test shows her vision to be 20/100 
in each eye. In the dark room the retinoscope shows a 
decided "against" the mirror motion of the reflex, which is 
stopped by a — 2 sphere. The ophthalmoscope shows the 
media to be clear and the fundus normal, the texture of the 
retina being visible at times with the aperture and again 
only with a — 1 or 2 D. lens. The ophthalmometer shows 
no astigmatism. When tested at the trial card, all convex 
glasses serve to make the vision worse, while with a — 2 
lens the vision in either eye is brought up to 20/15. One 
who was not reasonably accurate in the use of his ophthal- 
moscope, would naturally conclude that he was dealing with 
a true myopia, but the fact that the fundus was visible if 
only for a second, with the aperture, showed that the eyes 



HYPEROPIA. 141 

could not be actually myopic, while the fact that it was not 
always equally clear through the same lens showed a spastic 
condition of the ciliary muscle. There is nothing to be 
done with such a patient, but to examine again after 
several days' use of atropin. On the second examination 
the retinoscope, after making the correction for distance, 
shows a dioptre and a half of hyperopia which is confirmed 
by the ophthalmoscope. The patient's distant vision is now 
20/40, which with a + 1.50 is brought up to 20/20 in each 
eye. She fails to accept any higher correction in both eyes 
together, since no further relaxation is possible over that 
produced by atropin. This patient has overworked her 
ciliary muscle till it refuses to relax when the stimulus of 
near work is withdrawn. Consequently she has pain from 
the cramped muscle, while her crystalline lens has for the 
time become so convex that the rays cross in front of the 
retina instead of behind it. To prescribe the concave 
glass, because it improves her distant vision, would simply 
impose additional work on her accommodation and would 
very shortly increase her reflex pains at the same time that 
it still further reduced her ability to see without the glass. 
The atropin ought to be continued in her case till we are 
certain the spasm is overcome which sometimes takes weeks, 
and to prevent its return she ought to wear practically her 
full atropin correction constantly. 

The student will sometimes be in doubt, in cases where 
he has employed a cycloplegic and correctly estimated the 
total hyperopia, just what glass to prescribe. Some authori- 
ties prescribe practically the full correction in nearly all 
cases. The result is that in many cases the patient can, 
after emerging from his cycloplegia, see so much better 
without his glasses that he gradually discards them and 
perhaps entertains but a poor opinion of his oculist's skill. 



142 REFRACTION AND MOTILITY OF THE EYE. 

One should hesitate about ordering a full correction unless 
there is some clear reason for giving it, which the patient 
is sufficiently intelligent to understand. Some men do not 
order glasses till the patient has come out of the atropin 
and been tested a second time, when they correct the 
manifest hyperopia only. 

It is not to be forgotten that the amount of accommo- 
dation depends on two distinct factors : first, the elasticity 
of the lens and second, the actual muscular power of the 
ciliary muscle. The one fails regularly with age, but there 
is no reason for supposing that there is any material change 
in the other. The same muscular effort which at ten causes 
an accommodation of 15 D. ; at fifty only causes 2, but by 
using his full muscular power, the patient may have very 
distinct distant vision without any evidence of overwork. 
If, now, we prescribe a glass which makes the muscular 
effort unnecessary, the muscle rapidly loses power and the 
patient soon becomes absolutely dependent on glasses for 
distant vision as well as near. This is to be avoided in 
many occupations, as in the railroad service, where inability 
to see without glasses is ground for discharge. In such 
cases it is policy to prevent overstrain by a full correction 
for near work and at tfye same time to compel the eye as 
long as possible to maintain distant vision without glasses. 
In the same way the failure of vision after exhausting 
illness is due, not to any change in the lens, but to sheer 
muscular inability to place it in a position of complete 
relaxation. In many cases it is better policy, by a judicious 
admixture of rest, exercise and tonics, to restore the 
function rather than to prescribe glasses which will prevent 
it by making the muscular effort unnecessary. This is best 
accomplished by forbidding near work entirely for a time, 
by general building up of the bodily health, by very weak 



HYPEROPIA. 143 

eserin solutions, as ciliary tonics, and by under-correcting 
the distant vision or allowing no correction at all. In some 
cases, it is even advisable to have the patient wear very weak 
minus spheres for a short period every day to compel 
exercise of his ciliary muscles. 

As a rule we under-correct the total hyperopia, as 
estimated under atropin, by an amount which depends 
upon : — 

1. The age of the patient. The younger he is and the 
stronger his ciliary muscle, the more we can safely leave 
for it to do. 

2. The amount of manifest hyperopia. The less there 
is, the less of the total hyperopia we correct. As a rule we 
give a glass somewhat stronger than the manifest. 

3. The symptoms. When the patient simply com- 
plains of eyes tiring and needing rest only after long use, 
we correct but little more than the manifest error; but 
when he is subject to marked asthenopia, headache, neuras- 
thenia or other reflex symptoms probably due, to his eye 
strain, we correct a much larger part of his error, and when 
there is developing an actual squint or spasm of the accom- 
modation, we correct practically the whole error and, if 
necessary, insist on the patient's continuing his atropin till 
he can wear it comfortably. 

4. The patient's requirements. If his employment is 
such that he has an unusual amount of near work, we 
correct more of his hyperopia. 



CHAPTER VII. 

MYOPIA. 

Myopia is the term r.pplied to the refraction of an eye 
in which parallel rays of light tend to come to a focus before 
reaching the retina. The only rays which can focus 
properly are those which enter divergent and must there- 
fore come from an object at a lesser distance than infinity. 
Conversely, rays passing outward from the retina will 
emerge convergent. Evidently there are two factors which 
separately or together may produce this condition: either 
the refractive media of the eye are too strong, so that enter- 
ing rays are made to focus sooner than normal, or the 
retinal screen may be placed back of the normal focus of 
the media. Myopia which is due to an excess of power in 
the refracting media is known as refractive myopia, while 
that due to faulty position of the retina consequent to an 
increase in the antero-posterior diameter of the eye is known 
as axial myopia. 

Refractive myopia may be caused by a number of 
anomalies in the refracting media. For instance, an eye in 
which there is an abnormal convexity in the cornea with 
a consequent increase in its refraction would tend to be 
myopic, and, as we shall see, the convexity of the cornea as 
indicated by its radius of curvature is a very important 
factor in aiding us to decide whether a myopia is likely to 
be stationary or progressive. 

Any bulging of the cornea as the result of disease like a 
staphyloma would tend to make the eye myopic. Another 
important factor in refractive myopia is the lens. If the 
(144) 



MYOPIA. 145 

lens is abnormally convex or is of unusual density, its 
refracting power is increased, so that rays of light are 
brought to a focus before reaching the retina. This is very 
commonly noted in the early stages of cataract in which 
the lens swells, without as yet losing its transparency. In 
this condition the emmetropic patient becomes very myopic, 
is able again to read fine print without glasses and is said to 
have attained his "second sight," while the hyperope again 
sees distinctly at a distance without glasses and substitutes 
a weaker pair for his near work. This occurs so regularly 
that a boast of second sight should always make us sus- 
picious of incipient cataract. A dislocation of the lens so 
that it occupies a position nearer than normal to the cornea 
would tend to cause a myopia, especially marked, because 
with the tension of the zonula removed, the lens would 
become very convex. A much commoner form of refractive 
myopia is due to spasm of the ciliary muscle, by aid of 
which the lens becomes abnormally convex, and which 
persists as long as the spasm lasts. This is often due to 
faulty ocular hygiene, in holding print within a few inches 
of the eyes and then focussing on it, as many children do 
when they lie on the floor or sit with elbows on the desk. 
When they look up the ciliary muscle fails to relax and the 
eye remains apparently myopic. This may finally result in 
a real axial myopia. 

Axial myopia is due to an elongation of the posterior 
segment of the eye, with a consequent backward displace- 
ment of the retina. An eye with refractive myopia may 
generally be considered as an eye which is optically imper- 
fect, but perfectly healthy, while an eye with axial myopia 
is always a diseased eye, with potential dangers so great that 
special attention should be paid to its cause and the steps 
in its development. 



146 REFRACTION AND MOTILITY OF THE EYE. 

Myopia is only in the rarest instances congenital, since 
we have already seen that the great majority of children 
are born hyperopic. It only exceptionally occurs among 
savages or those who lead an outdoor life, while amongst 
civilized people it seems to be prevalent in exact proportion, 
to the amount of close work done. Among school children 
it is rare in the primary grades and increasingly common 
as the higher strain of education increases, so that in the 
high schools and colleges it is very prevalent. A study of 
different occupations shows the same thing. Almost 
unknown among farmers and outdoor workers, it is very 
common in factory workers, type setters and others, who 
are obliged to do close work continuously, each decade 
showing a larger and larger percentage of cases. It is 
notably more common among people who have wide fore- 
heads and eyes set far apart, and if any race has predis- 
position to it, it is the races of the German or Slav types, in 
which this form of skull predominates. While not con- 
genital, the tendency to the disease is without any doubt 
hereditary, since it very commonly occurs generation after 
generation in the majority of members in certain families 
in which the children, though born hyperopic, sooner or 
later become myopic. Few diseases have been more care- 
fully studied than axial myopia, and while the results are 
still in many respects indefinite, we have a working 
hypothesis of its development based on statistical informa- 
tion. 

In the broad-skulled individual of the G-erman type it 
is evident that since the eyes are set wide apart, they must 
converge much more than normal in doing the ordinary 
school or factory work at distances of ten or twelve inches. 
Consequently the eyes are subject to undue pressure from 
the extreme tension of the recti muscles. This alone should 



MYOPIA. 147 

tend to elongate the eye, but as we shall have occasion to 
notice repeatedly, there is a normal relationship between 
convergence and accommodation, so that the patient who 
has to converge too much, also accommodates too much. 

This takes place in our patient. When he converges 
his eyes on an object twelve inches away, the extra effort 
made necessary by the abnormal distance between his eyes 
is immediately responded to by an extra tension of the 
ciliary muscle, and his eyes, instead of being accommodated 
to an object at twelve inches, see it much better when it is 
brought up to ten. But when the object is brought up to 
ten inches, more convergence is necessary to keep both eyes 
on the same object. Thus is instituted a vicious circle, 
each effort at clear vision calling for an increase in con- 
vergence, while each increase in convergence is automatic- 
ally associated with an increase in accommodation which 
again calls for more convergence. This constant contrac- 
tion of the ciliary muscle not only tends to drag the walls 
of the eye toward the axis, but also by slightly compressing 
the contents to force the cornea and fundus further apart, 
and so of itself creates an axial myopia out of that which 
has so far been refractive. As the axial myopia develops, 
the patient's ability to see things close to either eye gets en- 
tirely beyond his power of convergence, and he gives up 
unconsciously all attempts at binocular vision and learns to 
suppress one image ; and as he is now so myopic that he can 
see without accommodating and is no longer bothered by 
diplopia, he ceases to converge. As we shall see in another 
chapter, the overworked muscles rapidly lose power, and 
very commonly a divergence of the eyes follows. 

But every pair of eyes in which there is an excess of 
convergence and accommodation does not develop an axial 
myopia, otherwise nearly every hyperope would eventually 



148 REFRACTION AND MOTILITY OF THE EYE. 

become a myope, which is very far from being the case. 
Evidently something further is necessary. The conditions 
referred to doubtless cause the tendency to elongation of 
the eyeball, but the elongation does not take place unless the 
tissues in the posterior half of the eye are softened by 
disease and so more susceptible to the stretching process. 
This tendency to scleritis and chorioditis, which is so com- 
mon in myopia, may be part of the individual heritage or it 
may be in part the result of the chronic congestion resulting 

from the constant strain 
of the eye. At times it 
occurs at the equator of 
the eye in the form of a 
scleritis, and the elonga- 
tion of the eye is due to 
a stretching here, but in 
the great majority of 
cases the change takes 
place about the posterior 
pole of the eye which 
keeps bulging further and further back into the orbit, the 
bulge constituting what is known as a posterior staphy- 
loma or sometimes as a myopic staphyloma. 

There are a number of anatomical changes character- 
istic of myopia. The antero-posterior diameter is so much 
increased that very frequently the eye seems to project 
forward from the orbit and be with difficulty covered by the 
lids, constituting what is commonly termed a "pop" eye. 
The pupil is very large and the patient commonly has a 
habit of squeezing his lids nearly together to exclude some 
superfluous light and make his vision better, by cutting out 
the diffusion circles in the same way that a pinhole disc 
would do. The anterior chamber is very deep and in well- 




MYOPIA. 149 

developed myopia each eye singly sees so much better, both 
near and far, with the ciliary muscle completely relaxed, 
that finally from sheer lack of use it becomes much smaller 
than normal. But the most marked changes are those 
which can be seen at the fundus with the ophthalmoscope. 
Very early there develops at the posterior pole and about 
the macula a condition of chronic congestion of the chorioid 
and retina which, instead of the clear, transparent red, 
shows a mixture of red and yellow and brown, which has 
been very aptly compared to the color of a ripe peach, and 
as the condition persists, more and more pigment is 
deposited, till finally we have the picture of a chorioiditis 
without exudation. 

Very early changes take place in the nerve head which, 
it will be remembered, enters somewhat to the nasal side of 
the posterior pole, and in the normal eye is practically 
perpendicular to the sclera at its point of entry. In the 
myope, as the stretching process proceeds, the posterior 
pole is pushed further and further backward till the nerve 
appears to enter nearly at right angles to its normal course, 
while at the same time the abnormal convergence in front 
causes an abnormal traction on the nerve behind. The 
nerve head instead of being in the same plane as the retina 
is pulled into a funnel shape, which points toward the 
outer side of the eye, having its inner side perpendicular to 
the observer and indistinct, while its outer side is very 
conspicuous. The blood vessels often have a sharp bend 
and appear to come from the inner side of the nerve 
instead of from the centre. Chorioidal changes begin at the 
outer side of the nerve head, first in the form of a yellowish- 
red border which finally results in a local atrophy of the 
chorioid, allowing the glistening white sclera to show 
through. This constitutes the so-called myopic crescent, 



150 REFRACTION AND MOTILITY OF THE EYE. 

or conus, which, to the beginner, appears as part of the 
nerve head, but is to be distinguished because it is gen- 
erally much whiter and very often has a border of brilliant 
black pigment. This crescent, beginning at the temporal 
side, sometimes extends all around the nerve head and 
broadens till the entire region is filled with an area of 
glistening white. This is sometimes erroneously called a 
staphyloma, which is, however, a very different affair. The 
true staphyloma generally occurs considerably to the 
temporal side of the disc, where a whole section of the 
sclera, thinner and softer than normal, gives way and 
forms a saucer-like depression, sometimes a third of the 
whole fundus being involved, and which is visible, not 
because of any changes in color, but by the change in the 
course of the vessels as they dip down into it. 

Under such conditions none of the tissues of the eye 
are properly nourished ; the retina partakes of the changes 
which take place in the chorioid and loses more or less of 
its delicacy of perception, while, as it is no longer held 
against the latter by the shrunken vitreous, it is especially 
liable to detachment, which is the final stage of many high 
myopias. Changes in the circulation make the myopic eye 
very liable to retinal and chorioidal hemorrhages, which 
are especially likely to occur about the macula. Floating 
opacities in the vitreous are very common sources of 
complaint. 

Symptoms. — The symptoms in myopia depend on the 
amount of error present, and whether we have to deal with 
the benign refractive type or the progressive axial form. 
The distant vision is always reduced in direct ratio to the 
amount of myopia. The myope of 2 D., without using his 
accommodation, can focus on his retina rays from an object 
twenty inches away and get a distinct image, but the rays 



MYOPIA. 151 

from an object farther away than this come to a focus 
more or less in front of his retina and so form on it 
diffusion circles which always results in indistinct vision. 
This point, which is also the point where emergent rays 
cross, is the myopic far point. Objects within his far 
point he can see by accommodating and if his ciliary muscle 
is normally strong, he is evidently able to bring objects 



8_onc/L , -fa^y f* 




X* 



Fig. 66. 

much closer to his eye and preserve distinct vision than the 
emmetrope. The closest point at which distinct vision is 
possible is the near point, and the distance between the 
far point and the near is his region of accommodation whicli 
differs materially from that of either emmetrope or 
hvperope, as shown in the diagram. Let us suppose the 
individual to be 20 years old, in which case he should have 
a ciliary power equal to about ten dioptres. 

The emmetrope, with his accommodation relaxed, has 
his eye adapted to parallel rays coming from infinity, while 



152 REFRACTION AND MOTILITY OF THE EYE. 

by exerting his ten dioptres of accommodative power, he 
can focus rays from a point four inches in front of his eye. 
His region of accommodation reaches from four inches to 
infinity. If he had 5 D. of hyperopia he could not focus 
parallel rays except by accommodating 5 D. and the remain- 
ing 5 D. would enable him to focus rays from an object at 
eight inches, but no nearer. Evidently his region reaches 
from eight inches to infinity. If he were myopic 5D., he 
could not see clearly beyond eight inches, even with his 
accommodation completely relaxed, and if we add to the 
5 D., which he has by reason of his myopia, the 10 D. of 
actual accommodative power, his near point is that of an 
emmetrope of 15 D., or practically 2% inches. In other 
words, his region of accommodation reaches only from 2% 
to 8 inches; but he has one advantage over the other two 
in that, though his distant vision is very poor, he can see 
close at hand with a much smaller expenditure of muscular 
energy. We have seen that at forty years of age the 
average person has a range, or amplitude, or power of 
accommodation, of about 4.5 D. which brings his near point 
up to about nine inches if he is emmetropic. If he is 
hyperopic 2 D., his reserve of accommodation is only 2.5 D., 
which leaves his near point at about sixteen inches and has 
long since driven him to wear reading glasses. The myope 
of 2 D. at forty has his near point at about six inches and it 
has not receded to nine inches till he is about fifty years of 
age, while in the higher grades of myopia he might never 
have to wear a convex glass for close work. In other words, 
his presbyopia is delayed, or, perhaps, does not occur at all. 
While it may be truly said that in the lower degrees of 
myopia the indistinctness of distant vision is largely com- 
pensated for by the ease of close work, and the postpone- 
ment of presbyopia, the case is very different when the 



MYOPIA. 153 

degree is higher and tends to become progressive. In many 
such eases distant vision is reduced to mere perception of 
form and motion, and the individual is doomed to a life- 
time of peering and blinking. Meantime the far point is 
so close to his eye that binocular vision is possible in 
reading only by the greatest effort. In the higher degrees, 
too, it often happens that improvement by glasses is not 
satisfactory, since the nutrition of the retina has suffered 
so by the changes in the fundus that it has lost a large part 
of its delicacy of perception. The myope is subject to 
constant annoyance from niuscse volitantes, or floating 
specks, which are particularly noticeable when the eye is 
tired and the gaze is directed toward a white surface. 
Xot infrequently the patient is conscious of the larger 
floating bodies in the vitreous which can be seen with the 
ophthalmoscope, and has blind spots of greater or less 
extent from chorioidal atrophy or hemorrhage or retinal 
detachment. 

Pain in the eyes, headache, and a whole train of reflex 
disorders which so often pursue the hyperope, are com- 
paratively rare in pure myopia, since no effort of his 
accommodation is of any use for distant vision or of any 
necessity for near. In many of the cases the extreme 
convergence necessary for binocular vision causes pain, but 
after a time, most myopes reach a working basis by 
abandoning binocular vision even though the eyes do not 
actually diverge. The chief source of pain in many cases 
is this : that the myope, like other persons, sees things close 
at hand more sharply when his ciliary muscle is under some 
tension, and consequently the tendency is to hold objects a 
little closer than is absolutely necessary and so throw work 
on the ciliary muscle which it is not capable of performing 
without effort. In such cases the muscle may respond to 



154 REFRACTION AND MOTILITY OF THE EYE. 

the demands and increase in strength, thus relieving the 
pain; it may break down entirely under the strain, in 
which case the pains are greater and more continuous, till 
use of the eyes becomes impossible; or it may get into a 
state of irritability and develop that spasm which plays such 
a part in the increase of myopia. 

Prognosis. — It is very important to be able early in 
life to form an intelligent estimate as to whether the myopia 
in a given case is real and, if so, whether it is benign and 
stationary or pernicious and progressive. In the first case 
there is no occasion for anxiety, but in the second, the 
whole trend of the child's career is changed, since every 
possible strain on his eyes must be avoided. His education 
must be limited both in amount and in kind and he will 
afterward be debarred from any occupation which involves 
continuous use of the eyes. 

In the first place : Is the myopia a real one ? We have 
seen that children are born hyperopic, that they become 
myopic gradually, and that in a hyperopic child a spasm of 
accommodation may at first simulate the myopia which it 
afterward frequently produces. A correction in such a 
case without a cycloplegic only increases the difficulty and 
no mild cycloplegic will be of avail. Let it be atropin and 
let it be used several days to be sure of complete relaxation. 
A careful study of the hereditary tendencies of the child, 
the width between the eyes, the amount of convergence 
necessary for near work, as well as of the medical history, 
should be made, since they all show a tendency one way or 
the other; but the two most important points are the 
measurement of the radius of curvature of the cornea and 
the examination of the fundus. It will be remembered 
that the average cornea has a radius of curvature of 7.65 
millimetres. If the radius of curvature is less than this, 



MYOPIA. 155 

it indicates a cornea with a sharper curve and greater 
refracting power, while, if the radius he longer, the cornea 
is flat and under-refracts. This radius can in each case be 
measured with great accuracy by the ophthalmometer of 
Javal, as explained in another chapter. A short radius in 
myopia indicates that the case is of the benign refractive 
type in which the rays are prematurely brought to a focus 
by over-refraction. A long or even an average radius, on 
the other hand, indicates that any myopia present must 
be due to the displacement of the retinal wall backward, 
which would indicate the progressive axial type. Examina- 
tion of the fundus is also of great value. If no changes 
are present in a patient with a short corneal radius it is 
perfectly safe to say that the case is a non-progressive one. 
If the cornea is a flat one, the absence of fundus changes is 
not so conclusive, since the myopia is evidently axial and not 
refractive, and the changes are likely to develop later; 
while, if the changes are present which indicate an inflam- 
matory softening of the structures at the posterior pole, 
the diagnosis of a progressive myopia can be made very 
positively. Without corneal measure of this kind the value 
of statistics of the results of various methods of treating 
myopia have a very problematical value, since we are in no 
position to even guess what proportion of the cases would 
have remained stationary without any treatment. 

Treatment. — Prevention is better than cure. We 
have seen that in predisposed individuals an over-use of 
the accommodation finally resulting in spasm is one of the 
early factors in the disease. This occurs not only in 
myopia itself, but in low degrees of hyperopia and especially 
of hyperopic astigmatism, and any indication of over- 
stimulation of the ciliary muscle of a child should be met 
by a careful correction of the refraction and, if necessary, 



156 REFRACTION AND MOTILITY OF THE EYE. 

a temporary relief from school duties. The same rules 
obtain with young factory workers, who should restrict the 
use of their eyes for close work as far as possible. 

When we place a concave glass of suitable strength 
before the myopic eye, the rays of light which, entering 
parallel, focus in front of the retina are made just 
divergent enough to fall on the macula. If we place a 
somewhat stronger glass before the eye, the rays are made 
so divergent that they tend to focus behind the retina, but 
the patient applies his accommodation to these divergent 
rays and makes them focus, till a glass has been reached 
which is beyond his power to overcome. Just as the dis- 
tant vision of the hyperope is better with a glass which only 
partly corrects his error and allows him to accommodate, 
so the myope sees very much better with a glass which over- 
corrects his error and so enables him to use his accommoda- 
tion. At first sight it might seem that this would do no 
harm, as long as it gives such good vision, but if we 
remember that the ciliary muscle is small in myopia, the 
keenness of distant vision would be preserved at the expense 
of fatigue, while the additional strain of near vision would 
often prove entirely beyond the individual's power, even 
if his myopia were of the benign type. 

Long series of cases have been presented to show that 
the complete correction of the entire error of refraction in 
myopia prevents its progression in the great majority of 
cases, and the tendency among leading ophthalmologists is 
to follow this rule. But we know that only a minority of 
myopes, after all, have the pernicious type, and till we have 
the records of a large number of patients in whom myopia 
was proven to be axial, together with the data of the 
age, hereditary tendencies, etc., we are not in a position to 
more than guess at the proportion of cases which have been 



MYOPIA. 157 

rendered non-progressive by full correction. Meantime 
there are some advantages about under-correction as well as 
exact correction in certain cases, and while a definite rule 
saves much thought and many mistakes to the beginner, the 
more experienced man is only embarrassed by it. One 
ought always to be conscious of the likelihood of over-cor- 
recting every case in which a cycloplegic has not been used, 
and while this in many cases has been proven harmless, in 
many more it is a grave error, especially as the myope does 
not ordinarily make much use of his ciliary muscle and is 
therefore but slightly incommoded by a cycloplegic. 

Estimation of Myopia. — We approximately estimate 
his visual capacity by having him read without glasses 
through the pin hole disc which excludes from the eye all 
but the unrefracted axial rays, while if he can read No. I J. 
type without glasses when held sufficiently close to the eye 
his macula must be functionally useful. After this pre- 
liminary test we take the patient to the dark room, and if 
not under a cycloplegic, we get him to relax his ciliary 
muscle by gazing off into the distance while we use the 
retinoscope. The weakest concave glass that stops the 
motion of the reflex is approximately the measure of the 
myopia. Xo pretense is made that this method is either 
absolutely reliable or accurate, but it is a convenient way 
of rapidly getting at a patient's condition, since it esti- 
mates the error with great rapidity and sufficient accuracy 
and saves time spent in experimenting at the trial case. By 
using the retinoscope, while the patient wears his own 
glares, one can tell at once whether there has been any 
material under- or over-correction. In elderly people, whose 
accommodation is no longer active, it is a very satisfactory 
method of making a final estimate of the refraction, while 
in the very high myopias it is, for reasons which we shall 



158 REFRACTION AND MOTILITY OF THE EYE. 

presently see, a very much more reliable method of measure 
than that with the ophthalmoscope, since the lenses are 
always placed at the same distance in front of the eye. 
Retinoscopy without a cycloplegic is easier in myopia, be- 
cause of the large pupil usually present. 

The Ophthalmoscopic Examination. — First, using a 
convex glass, we examine the media for opacities, try the 
pupillary reaction and then approaching very close to the 
eye we examine the fundus with the weakest concave glass 
with which we can see the tapetum of the retina or the 
small vessels in the neighborhood of the macula, this glass 
being the measure of the myopia. By using a stronger 
glass, one can see equally well, but it is done by using the 
accommodation and hence it is not the true measure of the 
refraction. We now examine the fundus carefully for any 
evidences of lesions, such as a crescent about the disc, 
chorioidal changes at the macula, a beginning staphyloma, 
retinal hemorrhages or detachment, making a careful note 
not only of their presence, but also of their extent, so that 
we can judge intelligently later on whether the condition 
has progressed or improved. In the high degrees of myopia 
the direct examination of the fundus is often difficult and 
one can get a much better general idea by the indirect 
method, which, because of the large pupil, is easier than in 
hyperopia. The magnification is, however, so much less 
that one is apt to overlook small but important details. 

In the lower degrees of myopia up to eight or ten 
dioptres, the measurement of the refraction with the 
ophthalmoscope is likely to be fairly accurate, but in the 
high degrees of twenty dioptres and more there is a great 
likelihood of error. For instance, a myopic eye of twenty 
dioptres has a far point of only two inches and would be 
corrected by a concave glass of 20 D. placed in contact with 



MYOPIA. 159 

the cornea, but the concave glass loses strength by being 
placed further away and has to be made correspondingly 
stronger. At a distance of 13 mm., or half an inch, which 
is the usual distance of spectacles from the cornea, it 
requires a glass of 30 D. strength to produce the same effect. 
If the glass is placed at a distance of 20 mm., or practically 
% of an inch away, the correction becomes 35 D. In using 
the ophthalmoscope, uniformity of distance is hardly 
possible, owing to the shape of the forehead of both patient 
and physician, and unconscious errors are unavoidable. 
The closer the observer brings his eye to that of the patient, 
the less the likelihood of serious over-estimate. 

We now examine the* eyes with the ophthalmometer of 
Javal, which gives us the radius of curvature and indicates 
the presence or absence of corneal astigmatism, at the same 
time giving very strong evidence as to the myopia being 
refractive or axial. 

We next examine the patient's vision at the trial case, 
always remembering that a patient with normal vision 
cannot be myopic. After making a note of the vision in 
the right eye,, we place before it a glass somewhat weaker 
than the one we expect the patient to use and gradually 
increase its strength till we find the weakest glass which 
will secure the maximum vision. Not being under atropin, 
the patient will use his accommodation and invariably 
prefer and see better with an over-correction of the error. 
When we have gone through the same process with the 
other eye, we let the patient use both eyes together, thereby 
relaxing the accommodation, and try to secure the same 
vision with a weaker glass before each. 

Treatment. — If the patient has come simply because 
his distant vision is defective and complaining of no head- 
ache or reflex disturbances, if his erroi is not high, while 



160 REFRACTION AND MOTILITY OF THE EYE. 

the retinoscope, ophthalmoscope and trial case show the 
same amount of myopia without astigmatism, and if the 
myopia is refractive and not axial, we can safely prescribe at 
this visit. On the other hand, if the patient has reflex 
trouble of any sort, a high error, with changes in the 
fundus, if there is any discrepancy between the diiferent 
tests, if the vision with glasses is unduly reduced, and 
especially if there are any indications of a progressive, 
axial myopia, the patient should be put under atropin and 
examined again. 

The strength of the glass ordered is a matter of 
individual judgment, and it is to be remembered that we 
are now dealing with simple myopia, which is a very differ- 
ent thing from myopic astigmatism. If the patient has a 
myopia of 1 D., he needs his full correction for distance, 
while for near work he can use his glass or not as suits him 
best. If his distance correction is 5 D., he will be obliged, 
without glasses, to bring fine print up to eight inches which 
is too close, while the use of his full correction for near 
work puts a strain on his hitherto unused ciliary muscle. 
If he is young and strong, he soon develops a normal power 
and can wear his glass constantly without trouble; but if 
older he must have the glass for near work reduced should 
he develop symptoms of overstrain. The same reasoning 
applies to the higher grades. They all need their full cor- 
rection for distance and in many cases are not injured by 
slight over-correction, while very commonly they are much 
more comfortable with a reduction for near work. 

If the myopia tends to be progressive, near work should 
be reduced as much as possible and in many cases inter- 
dicted altogether. 

Another very important point is that the myope almost 
always holds things closer to his eyes than is really 






MYOPIA. 161 

necessary, and so puts an undue strain on both his accom- 
modation and his convergence, and he should be made to 
practice complete relaxation by increasing his working 
distance to the extreme limit. 

In high myopias, with opacities in the media and 
changes in the fundus, the very great improvement both 
subjective and objective that follows the use of the iodides 
in the form of the ordinary mixed treatment, should be 
borne in mind. 

Another procedure that has been much more thoroughly 
tested in Europe than here, is the reduction of the myopia 
by the formation of a traumatic cataract by discission, with 
the subsequent removal of the lens after the method of 
Fukala. The advantages of the procedure are that in 
suitable cases it makes a complete abolition of the accom- 
modation by removing the lens, and lessens almost to 
nothing the convergence by removing the far point. It 
would seem hardly justifiable in eyes whose vision with 
glasses is good, and unduly dangerous in eyes already sub- 
ject to chronic chorioidal changes. The ideal case would 
seem to be the high myopia without serious fundus changes, 
but with indication of progression. 

The amount of reduction in the myopia by extraction 
of the lens is rather a complex problem. We know that in 
the emmetropic eye removal of the lens causes a hyperopia 
of about 11.5 D., which at the usual spectacle distance is 
compensated for by a -(- 10 D. lens. In myopia, if the lens 
was at a normal distance from the retina (the myopia 
being corneal), its removal would reduce the myopia 11.5 D. 
actually, which would be equivalent to about 13.5 D. 
estimated at the spectacle distance. Such a patient would 
after operation have a practical emmetropia for distance. 
If, however, as in most myopic eyes, the distance between 

li 



162 REFRACTION AND MOTILITY OF THE EYE. 

the lens and the retina is greater than normal, the removal 
of the lens would have a proportionately greater effect. As 
it is not possible to measure accurately these distances, the 
effect of the operation is rather uncertain, reducing the 
myopia between a minimum of about 15 D. and a maximum 
of 25 D. Therefore, if the myopia be less than 15 D., the 
operation is not advisable, since the patient would have to 
wear a convex glass for distance and another stronger one 
for near work, and his eye would be deprived of the 
adjustability for different distances which goes with ac- 
commodation. 

In prescribing glasses of all kinds, the physician should 
convince himself that the optical centre of the lens, as 
found on page 30, should be exactly in front of the centre 
of the pupil to avoid the prismatic action of the lens on 
rays which pass outside. This is much more important in 
myopia, since the average error is much higher than in 
other conditions. A patient wearing a — 10 D. lens, 
improperly centred, has to overcome a very powerful prism 
whose strength increases progressively from its centre. 



CHAPTER VIII. 
ASTIGMATISM. 

Astigmatism is the refractive condition of an eye in 
which rays of light do not come to a focus at any single 
point, owing to irregularities in the curvature or density of 
the refracting media. It is by far the most important 
and difficult refractive error with which we have to deal, 
because the majority of patients have it in greater or less 
degree and because, even when so slight as to cause no per- 
ceptible diminution in distant vision, it commonly entails a 
multifarious set of direct symptoms grouped under the 
term "eye strain" or asthenopia, and is secondarily the 
cause of a host of reflex functional troubles ranging from 
headache to the most intense mental depression. It is 
certainly capable of eventually causing organic changes in 
the eye, and many authorities are arguing that it may also 
result in organic disease in other organs. It is the most 
difficult subject in refraction. Astigmatism is of two kinds, 
regular and irregular. 

Irregular Astigmatism occurs when the curvature or 
refracting power in any one single meridian is not every- 
where alike, so that rays passing through it never focus at 
the same point. This is most marked in diseased condi- 
tions of the cornea. For instance, if the centre of the 
cornea is flattened or pushed forward, it has an entirely 
different refractive power from the periphery, and rays of 
light which pass through are broken up as they would be 
by a flaw in a window pane. When the cornea is studded 

(163) 



164 REFRACTION AND MOTILITY OF THE EYE. 

with numerous facets as the result of the contraction of 
ulcers in cicatrizing, the irregular astigmatism is still more 
marked, while after severe types of keratitis light may be as 
much diffused as is the sunlight in passing through a 
ground glass, in which every facet acts as an independent 
optical system. 

Some irregular astigmatism is present in the lens of 
every eye. The lens, it is to be remembered, is made up of 
several sectors reaching from the equator to the centre, each 
one of which differs slightly in curvature and refraction 
from its neighbor. For this reason rays from a point of 
light at infinite distance, like a star, are refracted unequally 
by the different sectors, though the variation is so slight 
that the images generally overlap and give the impression 
of a star instead of a mere point of light. If the refractive 
power or curvature of these sectors is abnormally increased 
by disease, one or more of the points of the star become 
absolutely distinct from the rest as a point of light, and 
monocular diplopia develops, or polyopia. This very 
often occurs in incipient cataract, when the sectors of the 
lens are unequally swollen, but are yet transparent. 

Irregular astigmatism reduces the vision and causes 
distortion of objects, which it is beyond our power to cor- 
rect by glasses. Sometimes vision is very much improved 
by placing before the eye an opaque disc with a pin point 
aperture, in this way reducing the illumination, but also' 
shutting out many rays which, by irregular refraction, 
would cause confusion. This is especially useful in kera- 
titis, after trachoma, or in corneal ulcers, where one or 
more small areas of healthy cornea are left. 

Irregular astigmatism, as the result of corneal scars or 
lenticular opacities, can be readily diagnosticated with the 
ophthalmoscope, using a strong convex lens and gradually 



ASTIGMATISM. 165 

approaching the eye, when the opacities appear black against 
the eye reflex. Transparent irregularities are best seen 
with the plane mirror in retinoscopy, when they appear as 
ill-defined, wavy lines or masses against the red background, 
while the reflex seems to consist of several portions, some 
of which move with the mirror and some against it. 

Eegular Astigmatism occurs when the refraction 
throughout any single meridian is the same. Such astig- 
matism is generally corneal and occurs when the curvature 
of the cornea is greater in one meridian than in another. 
As a rule the meridians of greatest and least curvature are 
at right angles to each other and are called the principal 
meridians. Such a cornea has a surface like the side of an 
egg or the bowl of a spoon, and it will be useful to study 
the path of rays of light after passing through such a 
medium. If we think of the spoon as standing vertically 
on its point, its sharpest curve will be in the horizontal 
meridian and the slightest in the vertical, these being the 
principal meridians, while the other meridians will rep- 
resent curvatures gradually decreasing from the sharpest 
to the slightest. If we place a cylinder with its axis 
vertical, we find that rays of light passing through it are 
bent toward the axis and come to a focus in a vertical line, 
called the focal line, and if we place another cylinder before 
the first one with the axis horizontal, the rays also are bent 
toward the focal line which is horizontal. If the cylinders 
are of the same strength, the focal lines must be at the 
same distance away, and evidently all the rays focus at the 
point where the focal lines cross each other, and the com- 
bined cylinders are equivalent to a spherical lens. If the 
horizontal cylinder is weaker, its focal line will be further 
away, but the rays still pass through both focal lines, but 
evidently have no common focal point, since the lines do 



166 



REFRACTION AND MOTILITY OF THE EYE. 



not intersect. The effect will be exactly the same, if the 
two cylinders be combined in a single lens having a greater 
curvature in one meridian than in the other. Such a sur- 
face is called a toric surface and corresponds to that of the 
cornea in regular astigmatism. 

Bays from an infinite point of light will be refracted 
by the vertical cylinder so as to pass through the vertical 




Fig. 67. 

line A, and by the horizontal so as to pass through the 
horizontal line C, while if a screen be placed at various 
points in the path, the image formed will be a circle, a 
vertical line, a vertical ellipse, a circle, a horizontal ellipse 
and a horizontal line. 

When the image of a point is a circle, it is known as 
the circle of least confusion. 

This accounts for the peculiar vision of the astig- 
matic, in whose eye the cornea is a toric surface and the 
retina the screen, the image formed evidently depending on 






ASTIGMATISM. 167 

the difference in the refraction of the two meridians and 
the distance of the screen. 

The image of a point, if the retina be situated at A 
would be a vertical line, at B a diffusion circle, and at C a 
horizontal line. If the object of regard be a vertical line, 
made up of an infinite number of points, each point will 
appear at A as a line, and as they overlap each other, the 
result will be a black, distinct vertical line; at C on the 
other hand each point will appear as a horizontal line and 
the composite result will be a broad indistinct vertical line 
composed of an infinite number of horizontal lines super- 
imposed. A horizontal line is most distinct at 0, and 
least so at A. If the object of regard be a + , it will 
appear at A' as §i| and at C as 4j||l while at inter- 
mediate points neither arm is distinct. If the cross is not 
vertically placed, the lines are nowhere distinct without 
tilting the head, which is a fact to be remembered in 
studying the effects of astigmatism. The astigmatic 
patient, therefore, sees some letters much better than others 
according as the lines of which they are made up accord 
with the axis of his astigmatism, and we always have reason 
to suspect this error when the patient regularly miscalls 
certain letters in the test chart, w T hile apparently reading 
without difficulty others of a very much smaller size. 

While regular astigmatism in the great majority of 
of cases is due to the cornea having a greater curvature in 
one meridian than in the others, it is evident that the 
same effect would be produced by inequalities in the curva- 
ture or position of the lens. Regular lenticular astigma- 
tism is generally attributed to an excentric or oblique posi- 
tion of the lens which is rotated slightly on its vertical 
axis, thus increasing the refraction of rays which enter in 
the horizontal meridian. According to Tscherning the 



168 REFRACTION AND MOTILITY OF THE EYE. 

lenticular astigmatism is slight, varying from .25 D. to 
.75 D., and as we have no clinical means of measuring it 
by itself, it is commonly disregarded. A very high degree 
of obliquity of the lens, like that caused by a partial dis- 
location, might produce a corresponding astigmatism. 

Causes. — Eegular corneal astigmatism is due in many 
cases to asymmetrical development of the eyeball. The 
orbit is narrower in the vertical diameter than in the 
horizontal, and the eyeball in conformity is slightly 
flattened above and below. It seems natural that the 
anterior corneal segment should have a slightly sharper 
curve in the vertical than in the horizontal meridian and 
this we find in the great majority of eyes. In some cases, 
however, the horizontal curve is the sharper, and an effort 
has been made to prove that all astigmatism in excess of the 
physiological type alluded to above, is the result of 
defective cranial development. Another factor that must 
not be lost sight, of is the effect of the continuous pressure 
of the upper lid, which would tend to increase the vertical 
curve. This is especially noticeable in myopes and others 
who habitually screw their lids together to avoid light and 
secure better sight, and the tendency would be increased by 
inflammatory conditions which both increase the weight of 
the lids and the photophobia. A tendency to a sharp 
curve in the horizontal meridian follows the over-use of the 
straight muscles of the eye, especially the internus in con- 
verging. One has frequent occasion to notice in using the 
ophthalmometer that the astigmatism sometimes varies a 
couple of dioptres, owing to the lid and muscle pressure in 
a nervous child, while there are many instances on record 
of change in the corneal curve as the result of the lowered 
pressure after a tenotomy for squint. Very high degrees 
of corneal astigmatism often result from operations on the 



ASTIGMATISM. 169 

eyeball. For instance, after an upward section of the 
cornea for cataract, the lips of the wound regularly heal in 
an overlap, causing a marked flattening of the vertical 
meridian with a relative increase in the horizontal curve. 
Most of this astigmatism disappears after the lapse of 
several months. 

Varieties of Astigmatism. — Since the vertical me- 
ridian has the sharpest curve in the great majority of cases, 
the variety is spoken of as astigmatism "with the rule/' 
while the variety in which the horizontal curve is sharpest, 




Fig. 68. 

being the exception, is spoken of as "against the rule/' 
Both are unfortunate and confusing terms, but seem to be 
firmly engrafted into our terminology. It is the custom 
to consider cases in which the meridian of sharpest curva- 
ture is nearer the vertical or nearer the horizontal, as being 
with the rule or against the rule, respectively. A much 
better way is to classify the astigmatism according to the 
axis of the meridian of greatest curvature. The ordinary 
trial frame in use has degrees marked on the circumference 
of each cell for this very purpose, beginning with on the 
right side of each, as shown in Fig. 68. Thus in both 
eyes the vertical meridian is at 90°, and the horizontal at 
180°. In the majority of cases the meridian of greatest 



170 REFRACTION AND MOTILITY OF THE EYE. 

curvature corresponds in the two eyes. If it lies at 90° in 
one, it is very apt to be the same in the other. If it is 
inclined toward the nose in one, it is apt to be so in the 
other and be recorded at 75° in the right and 105° in the 
left. Very often both meridians are inclined to the 
temporal side, the angle of inclination being equal in each. 
Astigmatism is also classified according to the refrac- 
tion of the different meridians. For instance, in an eye in 
which one meridian is emmetropic and the other hyperopic, 
the astigmatism is spoken of as simple hyperopic. If both 
meridians are hyperopic, but more so in one than the other, 
the astigmatism is compound hyperopic. If one meridian 
is emmetropic and the other myopic, the astigmatism is 
simple myopic, while, if both are myopic, but one more than 
the other, the astigmatism is compound myopic. Mixed 
astigmatism is present, when one meridian is myopic and 
the other hyperopic. In all these cases the amount of 
astigmatism simply measures the difference in curvature of 
different meridians. In any of these we can make the 
horizontal curve equal by adding a convex cylinder with its 
axis vertical (90°) or by subtracting from the vertical 
curve by a concave cylinder axis 180°, whichever gives the 

best vision. Therefore, con- 
vex cylinders at the axis 90°, 
or concave at axis 180°, indi- 
cate an astigmatism with the 
rule. Many writers consider 
that cases in which the axis of 
the correcting glass is within 
45 degrees of the vertical 
and horizontal, should be 
included, as shown in the 
Fig. 69. diagram. 




ASTIGMATISM. 171 

Vision in Astigmatism of Various Types. — A low 
degree of astigmatism is not incompatible with normal 
distant vision, but in the high degrees of whatever type 
the visual acuity is diminished, and since astigmatism is 
much more marked at the periphery than the centre of the 
cornea, vision is often very much better when the pupil is 
contracted than when it is dilated. This sometimes results 
in a patient under a mydriatic showing much greater 
astigmatism than before its use. Experiments have proved 
that the astigmatic eye sees best when the vertical focal 
lines fall on the retina and the individual will, so far as 
possible, endeavor to bring this about either by using his 
accommodation or by changing the position of his head or 
that of the object of regard, so that while the horizontal 
lines may be indistinct, the vertical ones shall be sharp and 
clear, which is a great help in reading. These vertical 
lines are focussed by the horizontal meridians of the cornea 
and the patient is able, by squinting the lids, to shut out 
the blurred image of horizontal lines which are refracted 
by the vertical meridians of the cornea and so greatly 
improve his vision. The student will notice that astig- 
matics do this constantly. 

The most difficult position for reading is the reverse, 
in which the horizontal lines focus on the retina and the 
vertical ones are blurred. In this condition the letters all 
seem wider than normal, and if the type is fine, one letter 
overlaps another and all are indistinct. 

Since even the slightest astigmatism tends to render 
some meridians indistinct, we have some difficulty in 
accounting for the keenness of vision often present, in 
other words, of understanding the mechanism of compensa- 
tion. It was and still is a popular theory that in 
astigmatism a partial asymmetrical contraction of the 



172 REFRACTION AND MOTILITY OF THE EYE. 

ciliary muscle occurred, sufficient to change the- refraction 
of the lens in one meridian without affecting the other, and 
so compensating for astigmatism as high as 3 D. Manj 
experiments tend to negative this theory, though there are 
some clinical facts than can hardly be otherwise explained 
Hardly more satisfactory is the hypothesis that the eye 
separately focusses each of the principal meridians and 
then unites them in a composite mental picture. Probably 
the best working theory is that the eye automatically 
secures the best vision by accommodating so that the 
vertical lines fall on the retina, while the blurred horizontal 
ones are interpreted according to the individual ability to 
decipher them by the aid of experience. Good vision in 
astigmatism is then always at the expense of ciliary effort. 
If this were the sole cause of the asthenopia so commonly 
present, it would be no greater than in hyperopia which is 
compensated for in the same way, while it is a notorious 
fact that a half dioptre of astigmatism will cause more 
symptoms than several of hyperopia. This is only to be 
explained on the ground of asymmmetrical action of the 
ciliary muscles or of fatigue from the continuous task of 
deciphering diffusion images. This plainly shows how 
much more important it is to correct astigmatism than 
simple spherical errors when both are present, especially 
when we remember that small errors frequently cause far 
more subjective disturbances than great ones. This seems 
paradoxical, but the patient with a high error soon learns 
by experience that he cannot by any effort of his own see 
distinctly, and gives up straining, while if the error be 
small the sharp vision which follows strain incites him to 
continuous automatic effort. 

The symptoms of astigmatism depend a good deal on 
the variety present as well as upon the condition and age 



ASTIGMATISM. 173 

of the patient. They are of the same character as those 
caused by hyperopia except that they are often of greater 
severity. 

Estimation of Astigmatism. — A large number of 
ingenious methods, both subjective and objective, have been 
devised, but it is much better that the student should have 
a thorough knowledge of the two or three reliable methods 
than to confuse himself over a large number which have 
become obsolete or are mere theoretical curiosities without 
a practical value of an} r kind. 

By all odds the most accurate and generally useful 
objective method is that by retinoscopy as explained in 
Chapter IV. It has the advantage of showing the amount 
of astigmatism in the refractive media as a whole and not 
of a single medium as does the keratometer, and it is 
especially useful in detecting the very low grade errors 
which cause so much trouble and are so difficult to discover. 
Its disadvantages are that its mastery requires much prac- 
tice, good eye sight, and perhaps a special aptitude; that 
its results are reliable only when the accommodation is 
completely under control, either by a cycloplegic or the 
management of the physician, and because of the handicap 
which peripheral refraction through a dilated pupil puts 
on the test. In suitable cases it should have the preference 
over every other method as being more accurate as well as 
more rapid. 

The ophthalmoscope, as explained in Chapter III, is a 
valuable means of estimating astigmatism of high degree, 
but is absolutely useless in estimating low errors, since 
accuracy depends on the observer's ability to relax his own 
accommodation, which is never absolute except in old age. 
There is no certainty that the observer is looking through 
the optical centre of the media which would be necessary 



174 REFRACTION AND MOTILITY OF THE EYE. 



for accuracy, and moreover, he generally studies not the 
macular region which is devoid of distinct vessels, but the 
retina near the nerve head which may be on an appreciably 
different level. It is chiefly valuable as part of a routine 
examination of the fundus and as a check on other tests 
when no cycloplegic has been used. In astigmatism of high 
degree it is reasonably near the mark, and an experienced 
observer can even estimate the axis of the astigmatism. 
An oval nerve head is always suggestive of a marked 
astigmatism. 







Fig. 70. 

We have seen that regular astigmatism is almost 
always du« to changes in the curvature of the cornea; 
consequently any means of estimating this surface in 
different meridians must be of practical value. Hence the 
usefulness of the ophthalmometer. Its chief disadvantages 
are that, since it only measures the anterior surface of 
the cornea, it fails to show an astigmatic error due to 
lenticular changes which sometimes neutralize and some- 
times increase the corneal astigmatism. Hence it often 
fails to show astigmatism which is actually present and at 



ASTIGMATISM. 175 

other times indicates astigmatism which is already neutral- 
ized within the eve. The error of the instrument is slight, 
but unfortunately it is just these small errors that it is 
most difficult and important to find. The large errors, on 
the other hand, are almost invariably corneal and in these 
the ophthalmometer is very serviceable, especially since in 
these a half dioptre more or less is not relatively very 
important. The instrument is chiefly useful as a check on 
other methods of observation and particularly in cases 
where, for one reason or another, no cycloplegic is needed. 

There are a number of instruments on the market all 
of which are based on the same optical principle, and 
though they vary in detail, an understanding of one is 
sufficient for all. 

For all practical purposes the an- 
terior surface of the cornea may be con- 
sidered as a convex mirror and in our 
study of the reflections from mirrors in 
Chapter I, we found that that from a 
plane mirror was the same size as the 
object, while a concave mirror magni- 
fied and a convex reduced it. The re- Fig. 71. 
flection of a white disc or circle in a 
concave mirror would be perfectly round, but larger, while 
in the convex it would be smaller, but still round. If, 
however, the mirror were more curved in one meridian than 
the other, the reflection of the circle would be no longer 
round, but oval, and the greater the difference in curve, the 
more oval the reflection. This was noted very early in 
ophthalmology, and corneal astigmatism diagnosticated by 
the disc of Placido, the patient being seated with his back 
to a window while the observer held the disc in such a way 
that he could see its reflection in the cornea and watch for 




176 REFRACTION AND MOTILITY OF THE EYE. 

any distortion through the aperture in the centre. This 
would show plainly any irregular astigmatism and also high 
degrees of regular astigmatism, but the flattening of the 
circle was so slight as to be imperceptible in slight varia- 
tions. The next step was to place in the aperture a prism 
so arranged that the observer would see a double reflection, 




the edge of one disc just touching the other. If the 
reflections were perfectly round, by rotating the instrument 
one circle would seem to roll around the other with their 
edges in perfect contact. If, however, the reflections were 
even a trifle elliptical, they would separate in the meridian 
of sharpest curve and overlap in the flattest. Even the 
slightest overlapping or separation would show some 
corneal astigmatism and it only remained to work out the 
details for observing and measuring the amount of varia- 



ASTIGMATISM. 



177 



tion to give us the several excellent instruments we have 
to-day. In the more recent ones, instead of a white disc 
we have two illuminated mires which indicate the location 




Fig. 73. 



of the periphery of the imaginary circle, thus, and as the 
instrument is rotated, if the circles separate, the mire with 
the step overlaps on the par- 
allelogram and each step of 
overlap indicates 1 d. of corneal 
astigmatism. For purposes of 
estimation the barrel of the 
instrument contains a convex 
lens of known strength, so 
that no clear image of the re- 
flection can be obtained till 
the lens is an exact distance 
from the cornea. The strength 
of the doubling prism is so 
arranged that at this distance 
a displacement of exactly 3 
mm. occurs, so that when the 
mires just touch, the imagin- 
ary circles evidently have a 
diameter of exactly 3 mm. Fig. 74. 




178 



REFRACTION AND MOTILITY OF THE EYE. 



The mires are so arranged on the arc that one or 
both can be moved along it till they are in exact contact. 
Given the size of the object as shown by the 
distance of the mires from each other on the 
arc, the radius of curvature of the arc, which is 
27 cm., the size of the reflection, which is 
always 3 mm. when the mires appear to touch 
each other in the reflection, and we can cal- 
culate the focal length of the corneal mirror. 
For instance, if, when the mires are horizontal 
and 20 cm. from each other on the arc, they 
appear to touch each other as in Fig. 73, we 
can estimate as follows : — 

The size of the object is to the size of the 
image as the radius of the object is to the focal 
length of the mirror, or 200 mm. : 3 mm. : : 
\ 2700 : ?, from which we see that the focal 
g length is 4.05 mm., and as the focal length of a 
mirror is just half its radius of curvature, this 
cornea has a horizontal radius of curve of 8.10. 
Knowing the radius of curvature and the index 
of refraction of the cornea, it is possible to cal- 
culate in dioptres the refracting power of any 
corneal meridian, and if astigmatism be present 
to estimate it in dioptres. This, however, has 
been done for us by the designers of the instru- 
ment and recorded in the scale on it. 

We have had occasion before to imagine 
the cornea as composed of two cylinders at 
right angles to each other. When the mires 
are in the first position shown in Fig. 76 A, 
and just touching, we are evidently measuring 
Fig. 75. the curvature and strength of the vertical 



Si 1 




ASTIGMATISM. 



179 



cylinder and we read it off on the dial as being 45 D. 
Xow, when we rotate the instrument to the second 




P'tg. 



position, we are measuring the horizontal cylinder and, 
causing the mires to again come in exact contact, we 
find the measurement to be 47 D. Evidently, the dif- 



180 REFRACTION AND MOTILITY OF THE EYE. 

ference or astigmatism is 2 D., which can be corrected 
by increasing the vertical cylinder by adding a plus 
2 ax. 90 or by diminishing the horizontal by adding 
a minus 2 ax. 180, according as the patient is hyperopic or 
myopic. But the two imaginary cylinders need not be 
at exactly 90° and 180°, and so each mire has running 
through it a black line which, in the meridians of greatest 
and least curvature, become exactly continuous with each 
other, thus indicating the position of each cylinder whose 
strength we can measure as above. 

If the anterior and posterior surfaces of the cornea 
were exactly parallel and its substance of the same refrac- 
tive index as the aqueous, we could consider the cornea 
and aqueous as practically a single powerful convex lens, 
and the ophthalmometer would be a very exact means of 
estimating its power. Unfortunately, however, even the 
anterior surface is not regular in its curve, being more con- 
vex at the centre than at the periphery; neither is the 
posterior" surface concentric since it always has a sharper 
curve (the cornea is thinnest in the centre) and not 
infrequently a separate astigmatism of its own which may 
increase or may neutralize that of the anterior surface. 
Then the aqueous humor has a slightly lesser density than 
the cornea and a very slight refraction takes place between 
the two. Consequently, even under ideal conditions, the 
method is liable to a regular error of from one-half to one 
dioptre, and many authorities, from a study of average 
posterior curvatures of the cornea correlated with experi- 
ence, make it a rule to deduct half a dioptre in all cases 
"with the rule" and add the same amount when "against 
the rule." 

This works out very well in a series of cases, but in 
the individual case it is often guess work. 



ASTIGMATISM. 181 

If instead of looking perpendicularly through a spher- 
ical lens it is rotated on its vertical axis and looked through 
slantwise the effect is that it has had added to it a -J- cylin- 
der, axis 90, and the more it is rotated the greater the 
astigmatism. The same thing holds true of the eye in 
which the visual line cuts the cornea to the inner side of the 
optic axis and is therefore not quite perpendicular to the 
refracting surfaces. Bays of light are therefore more 
strongly refracted in the axis of 90 than in that of 180, 
which would be equivalent to a slight astigmatism against 
the rule, the amount varying with the angle made by the 
opiic axis and visual line at their intersection at the nodal 
point, or the angle gamma. In case of a negative angle the 
astigmatism would still be against the rule. 

Also under the pressure of lids or muscles the entire 
corneal curve may vary several dioptres while one is looking 
at it, if the individual is nervous or his eye strained. 

Another disadvantage is that the ophthalmometer 
gives no information as to whether the astigmatism is 
myopic or hyperopic, since this depends as well upon the 
refractive power of the lens and especially on the length 
of the eyeball. To be sure, one might infer that a very 
short radius in a sharply curved cornea would go with 
myopia, but if the eyeball were short, the focal point might 
easily be behind the retina. This is what we commonly 
find in hyperopia. A flat cornea would seem to indicate 
a focal point behind the retina, but we find this same con- 
dition in progressive myopia, in which the posterior pole of 
the eye has been pushed backward, so that the ophthal- 
mometer simply indicates that one meridian is more curved 
than another, and they can be equalized by increasing the 
curvature of one with a convex cylinder or diminishing 
that of the other an equal amount by a concave one. 



182 REFRACTION AND MOTILITY OF THE EYE. 

Astigmatic Charts. — These are based on the fact 
that in astigmatism vision is clearest in one meridian and 
that, if a number of lines radiating from a common centre 
are viewed, one set should be much blacker and more dis- 
tinct than the others. In practice, however, many patients 
with a known astigmatism see all the lines alike, either 
because they cannot discriminate small differences or be- 
cause they instantly focus on the lines they are regarding. 
The smaller the pupil the greater the difficulty. On the 
other hand, some neurasthenics cannot be made to see all 
the lines alike by any possible nicety of correction. The 
clock dial chart is the commonest and may be taken as the 
type of them all ; and of the whole class it may be said that 
without cycloplegia they are absolutely unreliable and with 
cycloplegia unnecessary. 

Simple Hyperopic Astigmatism. — While subjective 
tests! should be dispensed with as much as possible in favor 
of objective ones in astigmatism as in other errors of 
refraction, the trial case test is the court of last resort, and 
as its use varies somewhat with the variety of error present, 
let us consider first a case of simple hyperopic astigmatism 
in a young person, the patient being seen for the first time 
and without a cycloplegic. The retinoscopic test reveals a 
simple hyperopic astigmatism of 1 D. axis 90. The 
ophthalmoscope shows the media clear and fundus normal, 
the disc very slightly oval perhaps. The ophthalmometer 
shows an astigmatism of 1.50 D., but without indicating 
whether it is plus at 90° or minus at 180°. When we seat 
this patient before the trial cards and test the vision in 
each eye separately, we find that his vision is 20/20 except 
that he miscalls a letter or two in many of the lines. We 
have already seen that, in reading, the vertical lines in the 
letters are more important than the others and in this case 



ASTIGMATISM. 183 

the vertical lines fall behind the retina, while the horizontal 
ones are directly on it. Instinctively he accommodates so 
as to see the vertical lines distinctly and so throws the 
horizontal out of focus and consequently confuses P, T and 
F, H, X and X. This alone should suggest astigmatism. 
We now place in the trial frame a + .50 at the axis 180°. 
This carries the horizontal strokes so far in front of the 
retina as to perceptibly diminish the vision, but as we rotate 
the glass in the frame toward the axis of 90 °, he is able to 
focus the vertical lines on the retina without accommodating 



+ 




Fig. 77. 



so much, and consequently his horizontal lines are not 
focussed so far in front, and his vision is perceptibly 
clearer. Eight here one should change the axis of the 
glass a few degrees on either side of 90°, to find if possible 
the position of sharpest curvature, taking particular pains 
to see that the patient holds his head squarely. If the 
astigmatism is exactly vertical and the glass is improperly 
placed, the tendency is for the patient to tilt his head till 
the glass is vertical and then rotate his eye in its socket 
till vision is best. The glass should fit the eye, and not the 
eye the glass. We now place stronger and stronger cylin- 
ders before the eye, till the patient begins to complain that 
his sight is being fogged. In this case this will probably 
occur with a -\- .75, axis 90°. Covering this eye, we go 
through exactly the same process with the other and dis- 



184 REFRACTION AND MOTILITY OF THE EYE. 

cover, let us suppose, exactly the same condition. We 
know that the patient has at least a dioptre of error in each 
eye, and so we try a + 1 ax. 90° in each, using both eyes, so 
relaxing the accommodation, and the binocular vision is 
still 20/20, though the patient plainly expresses a prefer- 
ence for a + .75 or perhaps a + .50. Here the wish 
of the patient plays an important part. If a sportsman or 
a surveyor with no complaint beyond a slightly indistinct 
vision of distant objects, it would be ridiculous to prescribe 
the full correction which would defeat the purpose of the 
consultation, and that glass should be given which gives the 
sharpest distant sight. If, on the other hand, the patient is 
a scholar or a bookkeeper who complains that his eyes tire 
after a period of use, and an occasional headache, it is 
easy to convince him with a book of Jaeger type that the 
+ 1 ax. 90, while not so distinct for distance, is very much 
more restful for near work and he can take your word for 
it that, as he gets used to it and learns to relax his strained 
ciliary muscle, his distant vision will also improve. 

If the patient has been referred on account of severe 
headache and some reflex trouble, if he has a heterophoria 
or there is a marked discrepancy between the subjective 
and objective tests which makes it advisable to get at the 
total error unmistakably, he should be put under a cyclo- 
plegic and examined again. This will show a slightly 
higher astigmatism + 1.25 by the retinoscope, the differ- 
ence depending on the observer's skill and experience in 
controlling his patient's accommodation without atropin 
in the first examination, and also on the presence in the 
first examination of lenticular astigmatism due to accom- 
modation which has disappeared under the cycloplegic. 
This is rare. 

The ophthalmoscopic and ophthalmometry findings 



ASTIGMATISM. 185 

will rarely vary much. With the trial case, however, we 
find the distant vision has fallen materially because the 
patient can no longer focus the vertical strokes on his 
retina by aid of his ciliary muscle and so only sees the 
unimportant horizontal strokes. A plus sphere of 1 D. 
will improve this vision materially, because it brings the 
vertical strokes to a focus on the retina even though it 
blurs the horizontal ones. A plus cylinder of 1.25 D. axis 
90, however, is much preferred, for it makes the vertical 
lines distinct while not reducing the horizontal, and gives 
a maximum vision of perhaps 20/20, but very likely only 
20/30, owing to the dilated pupil and consequent dazzling 
and peripheral refraction. If we rotate the cylinder either 
way from the vertical even a little, the vision is somewhat 
lessened and the patient complains that the letters are 
tipped to one side or the other slightly. 

If we increase the cylinder beyond + 1.25, the vision is 
materially reduced, also if we place even a + 25 sphere 
over the cylinder. We find the same condition in the 
other eye. If the patient* s condition is such that we wish 
him to wear the full correction, whether it reduces his 
vision or not we order it, but unless one has the confidence 
of his patient completely, it is advisable to continue the 
cycloplegic for several days after the glasses are first put 
on, as otherwise the patient will constantly see better with- 
out them and either desert entirely or discard the glass at 
every opportunity. If for any reason the patient cannot 
use a cycloplegic, I never hesitate, if he is intelligent and 
I have his confidence, to order what I think his full 
correction, no matter what the effect on distant vision, but 
it is often a hazardous experiment, The higher the 
astigmatism, the more it is ordinarily permissible to leave 
uncorrected. For instance, if a patient at the first 



186 REFRACTION AND MOTILITY OF THE EYE. 

examination and without atropin accepts a + 2.50 axis 90° 
and the subsequent test under cycloplegia shows a total 
astigmatism of 4.50, the glass ordered should be a 3.50 or 
even 4, depending somewhat on the patient's age: the 
older, the higher the acceptance. Patients with cyl- 
indrical lenses should always be cautioned about the 
probable apparent effect of these glasses, which, for a 
time seem to distort objects and distances in proportion to 
the strength of the glass. They seem to be walking up 
hill or the houses threaten to topple into the street, and 
they misjudge distances badly. This all disappears in a 
few days' time. 

Compound Hyperopic Astigmatism. — Let us suppose 
a young patient whose retinoscopic test without cycloplegia 
is -f- 3 -f- 3 axis 90°, while the ophthalmometer shows a 
refractive strength of 42 D. at 90° and 45y 2 D. at 180°, 
thus indicating an astigmatism of 3 D. "with the rule." 
At the test card the vision is considerably reduced since the 
patient has to accommodate 3 D. to see the horizontal 
strokes in the letters and 6 D. the vertical ones, and the 
astigmatism is so great that when either set is clear the 
other is too much obscured for accurate guessing. We 
are not surprised to find the vision 20/70 in each eye. We 
place a convex cylinder of 1 D. before the eye and rotate it 
to the axis where it gives the best result, which we find to 
be at 90°, and by gradual stages we increase its strength to 
2D., though the patient says he can see a trifle clearer with 
+ 1.50. As the strength of the cylinder increases, the 
patient becomes more intolerant of any deviation from the 
axis 90°. The use of this cylinder corrects a large part of 
the astigmatism and enables him by accommodating about 
3 D. to focus both the horizontal and vertical strokes so 
nearly together on his retina as to improve his vision up to 



ASTIGMATISM. 187 

possibly 20/30. If now we place in front of the cylinder 
a convex spherical lens, we do not necessarily improve his 
vision, but give him the same vision without compelling 
him to accommodate, and we increase the sphere till it 
begins to reduce his acuity. Probably he will not be able 
to relax so as to accept over + 1.50. We have then got him 
to accept a + 1-50 C + 2.00 axis 90° in each eye alone, 
while we feel confident that, if he could relax completely, 
it should be nearly +3 + 3 axis 90°. By using both eyes 
together we can certainly increase the sphere and possibly 




the cylinder without reducing his vision and can get him to 
wear a combination of + 2 + 2 axis 90° in each, = V 
20/30. If he consults us because of headache or tired 
eyes, we prescribe the strongest possible combination, con- 
fident that it will be more of a comfort for near work than 
for far. But if the glass is for the improvement of distant 
sight, he will be much better pleased, if the spherical 
element is reduced a little so as to let him exercise his very 
efficient accommodation a little. If it seems best to 
examine the patient under atropin, if our retinoscopy is 
accurate, he ought to obtain his maximum vision with 
— 2.75 + 3 axis 90 in each eye. When we test his vision 
without glasses, we find it greatly reduced, since he no 
longer has the asistance of his accommodation perhaps 
20/200 in each eye. For the same reason neither the 



188 REFRACTION AND MOTILITY OF THE EYE. 

cylinders nor spheres alone cause the improvement they 
did in the previous examination. The best way is to try 
him at once with the full retinoscopic correction and after 
testing the vision make slight changes in both sphere and 
rcylinder till the combination is found that gives the best 
vision. Owing to the dilated pupil one would hardly 
expect to get as good a maximum vision as at the first test. 
If the patient prefers a combination within a quarter of a 
dioptre of the retinoscopic finding, there is every reason to 
be satisfied; but if the beginner gets within a dioptre, he 
need not be discouraged. If, however, there is a marked 
discrepancy, it indicates a mistake in retinoscopy, or that 
the cycloplegia is incomplete, while children often memorize 
a test card and seem to see much better on a familiar one 
than they really do. Having estimated accurately the 
refraction, the final prescription depends on the judgment 
of the surgeon and the object sought by the patient. Get 
as near to the ideal as possible, particularly in the cylinder. 
The general rule in hyperopic astigmatism, whether simple 
or compound, would be: prescribe the strongest convex 
cylinder that the patient will accept without cycloplegia in 
any case and go beyond this if circumstances make it 
advisable. Be sure that the patient holds his head per- 
fectly straight during all tests and' arrange the axis of the 
cylinder in the position which gives the sharpest vision 
without any tipping or distortion of the test letters, even 
when it does not exactly agree with the axis indicated by 
the ophthalmometer and the retinoscope. After ascertain- 
ing the strongest cylinder, place over it, if possible, the 
strongest sphere which does not actually reduce vision. 

Myopic Astigmatism — Simple, — Suppose a young 
person with a simple myopic astigmatism with the rule of 
1 D. as shown by the retinoscope and ophthalmometer. 



ASTIGMATISM. 189 

When seated before the test cards, the vertical strokes in 
the letters are focussed on the retina, while the horizontal 
ones are focussed in front of it If the error was a very- 
slight one, the patient conld see enough of these latter to 
guess very accurately at the letters ; if the error is greater, 
vision is correspondingly uncertain, only a series of vertical 
black lines being perceived. Accommodation only makes 
matters worse by focussing both meridians in front of the 
retina. If we make the mistake of trying spherical lenses 
first and place before the eye a — ID., vision is improved, 




Fig. 79. 

because the horizontal strokes now focus on the retina and 
the vertical behind it, when by a slight exercise of accom- 
modation they can be seen. In other words, we have 
changed a simple myopic into a simple hyperopic astigma- 
tism, which has the advantage of giving him better distant 
vision, but causing him more strain. If, however, we sub- 
stitute a concave cylinder axis 180°, we carry back the 
horizontal focal line till it falls exactly on the retina 
without affecting the vertical which is already there. 
Consequently we have a maximum of vision with a 
minimum of strain. If the strength of the cylinder is 
increased beyond the full correction, the focal line is 
carried further behind the retina and the tendency is for 
the individual to strain his accommodation to bring it 
forward again: hence moderate over-correction does not 



190 REFRACTION AND MOTILITY OF THE EYE. 

necessarily reduce vision. In simple myopic astigmatism, 
however, this accommodation blurs one meridian in exactly 
the proportion that it clears the other, and so reduces the 
vision again, and over-correction is not as likely as in 
compound myopic astigmatism. 

In Compound Myopic Astigmatism, since both prin- 
cipal meridians focus in front of the retina, the vision 
is very much reduced. Suppose the objective tests show a 
myopia of 2 D. with 2 D. of astigmatism axis 180°. If we 
make the mistake of beginning with spherical lenses and 

place a — 2 sph., we shall find 
the vision much improved; if we 

4h^ > ^^^ I \N\ increase it to — - 4 sph., the hori- 
\^^^ — I I J zontal focal line has been carried 
back to the retina, while the ver- 
tical ones are behind it. In other 
words, the astigmatism is now a 
simple hyperopic of 2 D. with which a young, person may 
have a vision of 20/30. Evidently the glass would improve 
vision, but would cause strain. The best way is to begin 
with the cylinder, gradually increasing the strength, and 
searching for the axis which gives the best vision, and 
when increase of strength no longer causes improvement in 
vision, to add concave spheres of gradually increasing 
strength. If the combination of lenses which gives the 
best distant vision nearly coincides with the result of the 
objective tests, it may be prescribed, but such is rarely the 
case, since the patient almost invariably obtains the 
maximum vision with an over-correction which allows him 
to accommodate. A moderate over-correction of the 
spherical element does probably no harm in many cases, 
but the same cannot be said of the cylindrical which should 
be as exact as possible. In myopic astigmatism, as in 




ASTIGMATISM. 191 

simple myopia, the rule is to find the weakest glass which 
will give the best vision. For this reason the great 
majority of myopes should be examined under cyeloplegia. 
In contradistinction to the hyperope, who always sees poorly 
under a tropin, the myope whose vision is best with accom- 
modation relaxed not infrequently sees better and so sub- 
mits more gracefully to the necessity. Under these 
conditions the subjective and objective tests should cor- 
respond within a fraction of a dioptre, and it only remains 
to decide on the glass to be prescribed. 

In low and moderate errors it is certainly best to 
order the full correction for constant use and it is best 
put on while the patient is still under cyeloplegia. In 
very high myopia and especially when the patient has 
never worn glasses, it is sometimes advisable to under- 
correct for a time and even prescribe a weaker glass for 
near work. This is a matter of individual iudoment, and 
my experience is that surgeons who are themselves myopic 
commonly believe in under-correction. The astigmatism, 
however, should be fully corrected and the reduction, if 
made at all, taken from the spherical element. 

Mixed Astigmatism can rarely be satisfactorily 
corrected without atropin. Suppose a patient whose error 
is + 2 axis 90° with — 2 axis 180°. Plus spheres only 
reduce his already very bad vision. If, however, we try 
minus spheres, the vision immediately improves. With a 
— 2 sphere, for instance, the horizontal strokes focus on 
the retina, while the vertical ones focus behind it. In 
other words, we have changed the mixed astigmatism into 
a simple hyperopic astigmatism which is to be corrected by 
placing over the concave sphere the strongest convex 
cylinder axis vertical that the patient will accept. The 
difficulty is. however, that the patient without cyeloplegia 



192 REFRACTION AND MOTILITY OF THE EYE. 

invariably demands an over-correction of his myopic 
meridian and an under-correction of the hyperopic. This 
patient under atropin should see best with — 2 C + 4 
axis 90°, while without it he would have a much inferior 
acuity with about — 3.50 C + 4 axis 90°. 

It is much the best plan to examine all cases of sup- 
posed mixed astigmatism under atropin, because in many 
of them the myopic element disappears with cycloplegia. 
Except in extreme cases the full atropin correction should 
be ordered. 

The problem presented in the correction of astigma- 
tism is not difficult so long as 
the error is large enough to be 

+ «.vfc^^~~/~~Y^ detected. In these cases the 
r^s^ — _| ) J retinoscopic appearances are 
characteristic and plain, the 
work at the trial case is easy 
because the patient definitely 
prefers a certain glass at a fixed axis, and the regular error 
of the ophthalmometer is so small in proportion as to be 
more or less negligible. 

The detection of the very low degrees of astigmatism 
is very much more difficult, the difficulty increasing pro- 
portionately with the minuteness of the error and unusual 
positions of the axis. 

Many authorities argue that errors so small as to 
almost defy detection are of no importance and do not 
require correction, and doubtless this is true in many 
cases. When we are searching, however, for possible causes 
of reflex troubles manifesting themselves perhaps in other 
portions of the body, these small errors are of much greater 
importance. 

In my own experience I have often noticed that some 




ASTIGMATISM. 193 

of the most brilliant results followed the detection of 
astigmatism of less than half a dioptre in eyes that were 
otherwise emmetropic. 

Work of this kind cannot be done without a cycloplegic 
and is often very difficult with one. Under cycloplegia the 
retinoscope may show an astigmatism of less than half a 
dioptre but the presence of spherical aberration often makes 
the result very uncertain both as to amount and axis. The 
ophthalmometer is not more helpful for it was never 
intended for measuring such small fractions. It simply 
indicates that an error, if present, must be a very small one. 

The whole thing must therefore be worked out at the 
trial case with much patience, aiming to discover the com- 
bination which gives the sharpest and clearest distant 
vision. In these low astigmatisms it is often very difficult 
to decide on the exact axis of the cylinder, since the patient 
has almost the same visual acuity when the axis is changed 
ten degrees or more to either side. One can rotate the 
cylinder each way till the patient is sure his vision is 
slightly lessened and then take the meridian half way be- 
tween as the axis. Here, if ever, the astigmatic chart is 
likely to be useful as a check on other measures, since 
the pupil is widely dilated and the accommodation at rest. 
The estimation of astigmatism is facilitated by the "cross 
cylinder" consisting of a weak + cylinder combined with 
a weak — one, their axes being at right angles. When 
held in front of the trial glasses it increases one meridian 
and decreases the other, while the patient can note the re- 
sult much better than after a slower change of the trial 
lenses themselves. 



13 



CHAPTEE IX. 
PRESBYOPIA— ANISOMETROPIA— APHAKIA. 

Presbyopia.— We have seen in our previous study of 
the process of accommodation that in extreme youth when 
the lens is very elastic and can assume almost a spherical 
shape, the individual can get a sharp image of an object 
very close to him and that the nearest point which is 
consistent with distinct vision is known as the punctum 
proximum. As the years increase, the lens undergoes a 
physical process of sclerosis, beginning at its centre, which 
gradually diminishes the ability to become spherical and 
consequently the near point gradually recedes from the eye 
and in extreme old age may reach infinity. It makes no 
appreciable difference to a person whether the near point 
is at four or six inches, but there is a point beyond which it 
cannot recede without causing inconvenience and disability, 
and when this point has been reached, presbyopia, or old 
sight, is said to have set in. The age at which this occurs 
depends on a number of conditions. The laborer suffers no 
inconvenience till it recedes three or four feet, for that is 
his ordinary working distance, while the engraver or lace 
maker who works steadily at small objects at ten or twelve 
inches is inconvenienced very early. Health and race are 
also factors. The robust, healthy person may retain the 
elasticity longer than normal, while the individual who ma- 
tures early is apt to have a premature sclerosis of the lens. 

That we may have some standard we assume that 
presbyopia begins when the near point has receded so far 
that the individual can no longer readily distinguish fine 
print at the distance of ten inches, and to secure uniformity 
Jaeger long ago introduced a series of test types beginning 
with a very fine and progressing by gradual steps to a very 
coarse print. These gradations were not scientific, because 
they were printed from type of various sizes as found in the 
(194) 



PRESBYOPIA. 195 

ordinary printing office, and the steps from one set to the 
next were not by any means equal, but the objections were 
more theoretical than practical. Later on Snellen intro- 
duced a type on the same principle as his large wall type, 
each letter oeing of such size as to subtend a definite visual 
angle. This is more scientific, but has failed to displace the 
Jaeger type and for practical purposes they can be used 
interchangeably. 

The nearest point at which No. 1 Jaeger can be read 
is the individual's near point, and when in the process of 
age the point recedes beyond ten inches or 25 cm., presby- 
opia has begun. This requires exactly 4 D. of accommoda- 
tion and consequently when the amplitude is less than this, 
the patient is presbyopic. No individual can continuously 
employ his entire accommodation without undue fatigue 
and experiments show that about one-third must be kept 
in reserve. Therefore, the emmetrope of 40, who can read 
No. 1 Jaeger at 10 inches for a few moments only, can 
read ordinary print at the usual reading distance of 13 
inches without tiring, since this requires only 3 D. of 
accommodation. 

In the table of amplitudes (Fig. 38) given years ago 
by Donders we see that the emmetrope at the age of 40 
has 4.5 D. left and hence is on the verge of presbyopia, upon 
which he is generally well entered by 45. 

The presbyope first seeks to compensate for his defect 
by pushing his book further away, which makes the type 
look a trifle smaller, but more distinct. Next he begins to 
require larger print which can be held further away without 
looking smaller. Reading at night or in bad light allows 
his pupils to dilate and so increase the size of diffusion 
circles with consequent dim vision, so that he sees best 
with the light so placed as to fall directly on his eyes, 



196 REFRACTION AND MOTILITY OF THE EYE. 

contracting the pupil. This type of presbyopia is not 
painful, because it is due to simple lack of elasticity in the 
lens. There is another form due not to deficient elasticity, 
but to defective power in the ciliary muscle which entails 
the same strain as does hyperopia and astigmatism. This 
is not really presbyopia, but subnormal accommodation, 
but its effect on the visual capacity of the patient is the 
same, plus pain. The two may occur together and their 
treatment is the same, but the latter often develops earlier 
in life than true presbyopia, is not always a constant 
condition, since it may be the result of fatigue or ill-health, 
and may disappear with the improvement of the muscular 
tone throughout the system. It is to be remembered that 
the distant vision of the presbyope may be normal. 

The refractive condition of the patient plays a very 
important part not only in influencing the age at which 
presbyopia begins but the disability it entails. For 
instance, a hyperope of 3 D. requires that amount of 
accommodation for distant vision alone and when this is 
added to the amount necessary for clear vision at ten inches, 
it is evident that 7 D. is required and that as soon as his 
accommodative power falls below that, presbyopia has 
begun. By referring to the table we see that accommoda- 
tion has fallen to 7 D. at the age of 30. 

Myopia on the other hand postpones the onset of 
presbyopia. For instance, a myope of 4 D. has his far 
point at ten inches and can read the finest print at that 
distance without accommodating at all. Consequently he 
will never become presbyopic. 

Astigmatism very often complicates presbyopia. When 
hyperopic and of high degree, it diminishes the clearness 
of near vision, and since it calls for abnormal accommoda- 
tion it causes early presbyopia. The lower degrees have 



PRESBYOPIA. 197 

Hot so much effect in hastening the approach of actual 
presbyopia, but from the fruitless strain they put on the 
ciliary muscle in near vision, they develop an intolerance of 
near work, which is practically a presbyopia due to in- 
sufficient accommodative power. 

In myopic astigmatism the patient approaches his 
book till the myopic meridian focusses on the retina and 
then focusses the other by straining his ciliary muscle 
exactly as in hyperopic astigmatism. Therefore, myopic 
astigmatism, whether simple or compound, does not post- 
pone presbyopia to the same extent as does simple myopia, 
and is often associated with accommodative asthenopia and 
retinal fatigue. 

The Treatment of presbyopia consists in ordering 
a convex glass strong enough to bring the near point up to 
ten inches. This will vary in strength with the age and 
refraction of the individual. If he is emmetropic at the 
age of forty, he has a power of 4.50 D., which is more than 
enough without a glass. At 45 his power has fallen to 
3.50 and he needs at least half a dioptre more added in the 
form of a convex lens. At fifty his power has fallen to 
2.50 D. and needs to be supplemented by a + 1.50 convex 
lens. At fifty-five a + 2.50 is needed, at sixty a -f 3.50. 
Theoretically a presbyopic correction of about 4 D. should 
be all that would ever be necessary for an emmetrope, but 
as the emmetrope reaches the age of about sixty-five, 
he becomes slightly and increasingly hyperopic through 
changes in his lens. 1 

Roughly speaking then, the presbyopic correction of 



i The lens is composed of concentric layers of varying re- 
fract ivity, so that light undergoes refraction every time it passes 
from one layer to the next, but with age this layer becomes 
homogeneous, and the total refraction of the lens is somewhat 
reduced. 



198 REFRACTION AND MOTILITY OF THE EYE. 

an emmetrope is about 1 D. for each five years after reach- 
ing forty. It is important to remember that the presbyopic 
glass, while it brings the near point closer, also does the 
same with the far point, and that as accommodative power 
grows less and less the flexibility of the eye does the same, 
so that in selecting a glass, one has to take into account the 
usual working distance. For instance, at forty-five with a 
+ .50 glass, the patient can by relaxing see distinctly at 2 
metres, or eighty inches, and by accommodating can also 
see distinctly at 10 inches or at any point between the two. 
At sixty-five, however, with a + 3.50, he cannot see beyond 
eleven inches, nor closer than 10. Consequently, the older 
the patient the more carefully one must consider the dis- 
tance at which he works. The carpenter or blacksmith at 
sixty-five could not wear the full correction because he can- 
not bring his work so close to his eyes. Either would be 
more useful with a + 2, while the student or the sewing 
woman, whose work is always close at hand, needs the full 
amount. 

If the patient is not emmetropic, he must be made so 
by glasses before estimating his presbyopia. In other 
words, we add to his full correction for distance about 1 D. 
for every five years of age after forty, which will generally 
enable him to read No. 1 Jaeger at ten inches. If it does 
not, one would suspect that the estimation of the distant 
vision was not correct, that a misstatement of age had been 
made or that there was some defect in the eye in the nature 
of a paralysis of accommodation or some change in the 
media. 

If the patient be hyperopic or astigmatic, the full 
correction for distance will generally be sufficient for near 
work as well till between forty and forty-five, after which 
two pairs of glasses must be used, one for near and the 



PRESBYOPIA. 199 

other for distant vision. The latter will require little, if 
any, change for many years, but the former will need to be 
increased at frequent intervals depending on the age and 
the rapidity of the failure of accommodation. Some 
individuals can go four or five years without change, if the 
full correction has been made and the work is not arduous, 
while many others are better for a slight increase every 
year or two. 

In myopia and myopic astigmatism the same rule 
holds good: the convex glass for near, neutralizing the 
concave distance glass to which it is added. For instance 
a myope of 2 D. at forty-five years would theoretically wear 
— 2 -\- .50 = — 1.50 D. for near work, which would be 
gradually diminished, as time went on. Practically, how- 
ever, such a myope may be without any glass at all up to 
fifty-five or sixty and would certainly object to wearing 
glasses for near work. In case of higher myopia of 8 or 
10 D. the patient's far point without glasses would be so 
close to the eye, 4 or 5 inches, as to be very fatiguing. If 
young, he would wear his full correction for both near and 
far, while if older, his presbyopic correction would take the 
form of a reduction in the strength of his glass. 

The ordinary presbyopic spherical lenses should be 
tilted forward so that, in his ordinary reading position, the 
patient looks perpendicularly through the lens. Other- 
wise it has the effect of a weak cylinder added to the sphere. 
For this reason some presbyopes with a low astigmatism 
against the rule are more comfortable without the cylinder 
a> they get the same effect by looking obliquely through 
the lens. 

A very convenient form of glass in presbyopia is the 
so-called bifocal in which the correction is cemented on to 
the lower half of the patient's distance lenses, so that when 



200 REFRACTION AND MOTILITY OF THE EYE. 

the patient looks off into the distance, he is using the upper 
half and when he looks down as in reading he looks through 
the stronger lower half. Such lenses require very careful 
optical adjustment, so that the patient may not be con- 
scious of the dividing line between the two parts and some 
nervous people never can learn to be comfortable in them. 
When properly adjusted, however, he cannot strain his 
accommodation for near work or for distance, and it is pos- 
sible to increase the presbyopic correction from time to 
time without changing the distance glass. 

Instead of the cement bifocal one of the various forms 
of ground bifocal may be used, the reading segment of 
which is practically invisible. Patients who do not need 
distance glasses and whose work requires both far and near 
vision, can have the upper part plane or have it cut away 
entirely, wearing a crescent shaped ^'clerical" or "pulpit" 
glass. In myopes this is often reversed, the upper part of 
the lens being retained for distance, while the lower part 
is cut away. 

Anisometropia is the term used to describe the 
condition of eyes whose refraction is unequal. In the 
narrow sense of the word no two eyes are exactly alike, but 
the term is applied only when the difference is great 
enough to be of practical importance. 

Anisometropia is often a congenital condition, one eye 
being smaller or with differently curved refracting surfaces, 
and not infrequently the inequality of the eyes is asso- 
ciated with unequal orbital or cranial growth. It may be 
also due to the varying progress of disease as in myopia, to 
operation or injury, as after cataract extraction, displace- 
ment of the lens, displacement of the pupil and changes in 
the corneal curvature and other similar causes. 

When uncorrected, it entails a series of symptoms 






ANISOMETROPIA. 201 

depending on the variety and degree of the refractive 
difference which are not infrequently replaced by others 
just as annoying after correction. The condition therefore 
often calls for the exercise of judgment and skill as to 
whether the individual will benefit at all from correction 
and, if so, whether the correction should be complete or 
only partial. 

Symptoms. — If the refractive difference is a material 
one, the image formed on one retina must be very much 
more indistinct than that formed on its fellow. Many 
authorities assume that some individuals, if not all, can 
compensate for this ametropia and make the refraction of 
both e}'es the same by an equal contraction of the ciliary 
muscles. In this case true binocular vision might be 
present. On the other hand there is much evidence that 
in many other cases the ciliary muscles must receive the 
same amount of innervation and that any change in the 
refraction of one eye must be accompanied by an equal 
change in its fellow. In such cases the retinal image in the 
worst eye must be not only dimmer, but must also vary 
slightly in size or form from its fellow, and while the 
brain has learned to form tolerably correct judgments from 
superposing one on the other, and it is a great help in 
stereoscopic vision, still vision is not in the true sense 
binocular. When the anisometropia is a slight one, the 
difficulty is negligible, but in many other cases binocular 
vision is maintained at the expense of constant ciliary 
effort, or the brain is fatigued by a continuous series of 
unconscious mental judgments as to the apparent and 
actual size, shape and distance of objects. Naturally this 
is often accompanied by the varied symptoms of ocular and 
nerve fatigue. 

In other cases of greater inequality the retinal images 



202 REFRACTION AND MOTILITY OF THE EYE. 

are so dissimilar that the fusion sense is either weakened 
or entirely abolished. Such patients very easily develop 
strabismus. 

It occasionally happens that the patient can use one 
eye for distant and the other for near vision. For instance, 
if one eye has 4 D. of hyperopia and its fellow an equal 
amount of myopia, the first would certainly be used for 
distance, while the other could be used for near work with- 
out any accommodation at all, but the vision would be in 
no sense binocular. 

The results of correction of anisometropia must also 
be borne in mind. In suitable cases of low degree in the 
young, distinct vision in both eyes results and the retinal 
images being sharp and equal in size and shape, perfect 
fusion occurs at once. In others the effect of the correction 
is in itself a source of confusion and annoyance. For 
instance, a patient has become accustomed to base his 
mental estimates on the distorted image of an astigmatic 
eye. He knows that objects which are to a certain extent 
oval, are actually round, and they finally cease to appear 
oval and the distorted image impresses him as regular. 
Now if this astigmatism is corrected so that the retinal 
image is actually correct, he has to revise his whole set of 
visual judgments. If he is young and plastic, this is easily 
done, but at the other extreme of life it is a great 
annoyance. 

The effect of lenses on the apparent size of objects is 
also to be remembered. For instance, if one eye be emme- 
tropic, while the other is highly myopic, the concave glass 
being a sensible distance from the eye makes the image on 
the retina actually somewhat smaller than in the other eye 
and apparently very much smaller indeed. The patient 
therefore, has two images which are sharp and distinct, but 



ANISOMETROPIA. 203 

of notably different size, and he has great difficulty in 
fusing them. If he has any latent muscular error, he has 
at once a diplopia which will result in fusion by straining or 
a squint by the suppression of one image. 

If the ametropic eye has a high degree of hyperopia, 
the convex lens causes an apparent increase in the size of 
the object of regard, and the same difficulty is met with. 
Hyperopia, however, being usually of low degree, is not 
often a source of trouble except after removal of the lens 
in cataract, which requires a correction of about 10 D. 

In this condition, if the other eye has good sight, the 
patient is often much more at ease if the aphakic eye be 
left uncorrected and used, not for central, but only for 
peripheral vision. 

Another source of trouble is the prismatic effect of 
strong lenses. A myope in converging looks through his 
concave glass to the inner side of the optical centre and it 
has the effect of a very strong prism, base in, the converse 
being the case in the hyperopic eye. When the patient looks 
above or below the centre of a strong lens, the prismatic 
action is often enough to cause an actual diplopia. It can 
be readily seen that the correction of anisometropia is a 
matter calling for the exercise of skill and judgment. 

In young persons, where the inequality is of low 
degree, it can usually be accomplished easily and will prove 
very beneficial, and even when the inequality is great a 
persistent effort should be made to secure binocular vision. 

In young adults whose mental habits are more firmly 
fixed, the prognosis is not by any means so good, while in 
the middle aged and the old, binocular vision in marked 
nni-ometropia is almost hopeless, and correction of the 
worse eye so likely to be the cause of fruitless annoyance as 
to be better left untried. 



204 REFRACTION AND MOTILITY OF THE EYE. 

Aphakia is the refractive condition of an eye whose 
lens has been removed or dislocated. In this condition the 
accommodation power of the eye has disappeared entirely. 
For distant vision the human lens in a normal eye can be 
substituted by a convex lens of about 10.5 D. worn at the 
usual spectacle distance. In both hyperopia and myopia 
of the refractive type, the measurements of the eye being 
otherwise normal, approximately the same correction will 
be called for. In axial myopia, however, as we have seen in 
another chapter, the effect of the removal of the crystalline 
lens varies widely according to the amount of elongation of 
the eye ball and the distance of the lens from the retina. 
The distance of the correcting lens from the eye is also a 
factor. Cataract extraction in high -myopia may therefore 
cause a refractive change seldom less than 15 D. and 
occasionally as great as 25 D. 

The fitting of cataract glasses differs somewhat from 
that of ordinary glasses. 

Theoretically the aphakic eye being without accom- 
modation should be fitted as exactly by retinoscopy as the 
atropinized eye, but practically it often happens, that the 
pupil is small or that there are extensive membranous 
remains with an opening large enough to give the patient 
clear vision, but altogether too small for successful 
retinoscopy. 

The same difficulty also occurs in the estimation of 
refraction by the ophthalmoscope, though in many cases it 
is possible. 

The ophthalmometer of Javal is a very reliable means 
of estimating the astigmatism, since in the absence of the 
lens it must be corneal. 

The usual section of the cornea in cataract is made 
upward, and in the process of healing the flap overlaps, 
causing a flattening of the vertical meridian with an 



APHAKIA. 205 

astigmatism against the rule, which may amount to 5 or 
6 D. or even more. A large part of this usually disappears 
within a few months; hence the first glasses are to be 
regarded as a temporary expedient. The ophthalmometer, 
therefore, affords a very accurate method of estimating not 
only the amount of astigmatism, but also its axis. 

When it comes to the trial case test, the patient should 
be given the glass or combination which gives him the best 
vision, since he has no accommodation and therefore cannot 
strain the eye. The patient must be told that he has to 
learn to use his eyes all over again. Familiar objects all 
look larger than before, and since the patient knows from 
experience how they ought to look, they impress him as 
being closer than they really are. He must be cautioned to 
move his head rather than his eyes in looking at things, so 
as to look through the centre of his lenses and avoid the 
cylindrical and prismatic effect, which is considerable. 

Being without any accommodative power, the aphakic 
eye must have an additional correction for near work. For 
reading, which is generally done at ten inches a + 4 D. 
should be added to the distance correction, many patients 
being able to wear bifocals with comfort. If the working 
distance be somewhat greater, the correction should be 
correspondingly less. The patient can see distinctly at 
only one distance, but he can, by sliding his glasses down 
on his nose, increase their power and so obtain a limited 
accommodation effect. It is not always desirable to correct 
the entire astigmatism against the rule, since the patient by 
looking obliquely through a spherical lens gets the same 
effect and is better pleased. 

When only one eye has useful vision, a reversible 
spectacle is very convenient, which contains the distant 
correction on one side and the near one on the other. 



CHAPTEE X. 

BINOCULAR VISION. 

We have hitherto treated the function of vision as 
though it were carried on with one eye, only incidentally 
referring to the fact that we have two. 

We have now reached the point where we must take 
cognizance of the fact that human vision is binocular and 
that the instinct for binocular single vision is imperious 
and requires a delicate and far-reaching co-ordination of 
nerve, muscle and brain. So imperious is this desire and 
so delicate the mechanism, that obstacles which impede or 
prevent binocular vision not only diminish the visual 
capacity, but also often cause a complicated train of reflex 
symptoms in other parts of the body. 

In one eye alone the only object which is seen with 
absolute distinctness is that whose image is formed at the 
macula; consequently, in order to see distinctly the eye 
is so directed that this image falls on the macula, whence 
the visual impression is transmitted to the brain. In using 
both eyes together, if they be equally good, they must be 
so turned that the image of the same object is focussed on 
each macula, when both transmit the same impression to 
the brain and satisfactory vision results. If, however, the 
eyes do not co-ordinate, the brain receives the impression 
of the object on one macula, but also at the same time an 
equally distinct impression of whatever other object happens 
to focus on the other macula at the same time. One learns 
in time to make a choice between the pictures and suppress 
the other, as in using the microscope with both eyes open ; 
(206) 



BINOCULAR VISION. 207 

but till this is done, endless confusion is caused. Evidently 
the two maculae, if they receive the same image and trans- 
mit to the brain a single impression, may be said to be 
"corresponding" or "identical" points. If we gaze straight 
in front, any object to the right falls on the left side of 
each retina, but as the brain receives the impression of only 
one, the points on which these images fall must also be 
identical points. The same is true of objects to the left 
or above or below the direction of the gaze. Evidently each 
retina is made up of points which correspond or are 
identical with points in the other, and we can easily 
determine these points by tracing the paths of light which 
fall on them from any object, the corresponding points of 
the two retinas being those on which the two images of the 
object fall when the visual axes converge at the object. 
Tracing this out, we find that the upper half of one retina 
corresponds to the upper half of the other. The lower 
halves correspond also, while the nasal half of one corre- 
sponds with the temporal half of the other. 

If the visual axes are not so directed that the image of 
the object of regard falls on identical parts of both retinae, 
a double picture is formed. For instance, if the object is a 
candle flame, which is focussed on one macula, but falls on 
some non-identical part of its retina of the other eye, the 
brain receives from the macular image the impression of 
one flame which is sharp and distinct, while from the other 
eye it gets the impression of an entirely different flame 
which, being received from a less sensitive part of the 
retina, is less distinct, but nevertheless very confusing. 
Since in the ordinary movements of the eye we see images 
singly, it is obvious that the movements of the eyeballs 
must constantly be so coordinated that the images of 
external objects fall on corresponding points of the two 



208 REFRACTION AND MOTILITY OF THE EYE. 

retinae. That we may have a clear conception of the way 
in which the coordination is obtained, we must study the 
movements of the eyeball and the muscles which control 
them. 

The eyeball in its orbit corresponds very closely to a 
ball and socket joint in which the only possible movements 
in health are those of rotation, and these rotary movements 
are carried on about an imaginary point in the centre of 
each eye, called the centre of- rotation, which has been 
experimentally located in the vitreous about 13.5 mm. 
behind the anterior surface of the cornea. It is, of course, 
quite different from the optical centre or nodal point of 
the eye. 

The imaginary line passing from the macula through 
the nodal point of the eye to the object of regard is known 
as the visual line of the eye. 1 When we look straight 
forward, the visual axes are parallel, and when we look at an 
object nearer than infinity, the visual axes converge more 
and more as the object of regard is nearer. The horizontal 
plane in which the visual axes lie is called the visual plane, 
and the vertical plane midway between the eyes is called the 
median plane. 

The primary position of the eyes may be described as 
that which is assumed when, with head erect, we look at an 
object infinitely distant on the horizon, when the visual 
axes will lie in the horizontal plane and be parallel to each 
other and the median plane. All other positions of the 
eye are secondary positions. We can conceive of three 



i This is not to be confused with the optic axis which is the 
line on which the cornea, lens and other dioptrie media are cen- 
tered. This passes through the summit of the cornea, the nodal 
point of the lens, and may or may not pass through the macula. 
In the perfect eye the optic axis would coincide with the visual 
axis, but it very seldom does so exactly ( Fig. 30 ) . 



BINOCULAR VISION. 209 

possible movements or combinations of movements of the 
eyes : first, lateral motion in and out as in adduction, 
abduction or convergence; second, vertical motions in 
raising or lowering the eyes; or combinations of the lateral 
and vertical movements. If we examine carefully, we 
shall see that no voluntary combination can be executed 
which causes the image of the object of regard to fall on 
portions pf the two retinae which do not correspond; in 
other words, we cannot make any motion of the eyes 
voluntarily which results in diplopia. We can move them 
to right or left and up or down in many combinations, but 
we cannot diverge them or direct one up and the other 
down, since these images would fall on dissimilar parts of 
the retina, and diplopia result. 

Some of these motions we can make involuntarily, but 
only when conditions are so arranged that no diplopia 
results. For instance, if we place prisms before the eyes 
in a suitable manner, we can cause the eyes to diverge, 
laterally or vertically, to a certain extent, because the 
prisms change the position of the images so that they 
continue to fall on "identical" portions of the retina in 
spite of the divergence. 

There is a third motion of the eye, that of rotation 
about its optical axis as a wheel rotates on its hub. If the 
two eyes are rotated wheel-fashion, so that their vertical 
planes are inclined toward each other above (intorsion) , the 
image of the object of regard, a vertical line, for instance, 
instead of falling on the vertical meridian of each retina, 
would lie in the nasal half of each below, and in the 
temporal half of each above. These are not corresponding 
points in either case and the vertical line would appear like 
an oblique cross. Similar phenomena would occur if the 
vertical planes were inclined away from each other, or if 



210 REFRACTION AND MOTILITY OF THE EYE. 

one were rotated in either direction while the other 
remained perpendicular. If, however, both planes are 
rotated equally in the same direction, the image continues 
to fall on identical areas and no diplopia results. In other 
words, the perpendicular planes of the eye, if rotated, must 
be kept perfectly parallel to avoid diplopia. Inasmuch as 
we do not regularly have diplopia, it is evident that these 
planes are kept parallel throughout all the various move- 
ments of the eyes. The numerous possibilities of diplopia 
in binocular vision indicate not only the imperious instinct 
for single vision and the very delicate muscular and nerve 




Fig. 82. 

coordination necessary to maintain it, but also indicate the 
tremendous struggle on the part of nature to maintain it 
when congenital or acquired obstacles are interposed. 

Now let us consider for a moment the different 
muscles which move the eyeball, their action and the means 
by which they are coordinated. Each eye is supplied with 
four straight muscles and two oblique. The straight 
muscles all have their origin from the bony walls at the 
apex of the orbit. The internal rectus passes forward 
horizontally along the inner wall and is inserted into the 
sclera about 5.5 mm. from the margin of the cornea, the 
line of insertion being vertical. Its action is to turn the 
cornea horizontally toward the nose. The external rectus 
is a much smaller muscle which passes horizontally forward 



BINOCULAR VISION. 



211 



to the outer side of the ball and is inserted into the sclera 
6.9 mm. from the margin of the cornea. Its action is to 
turn the eyeball horizontally outward, being directly 
antagonistic to the internus. The superior rectus is a small 
muscle which runs forward and outward and upward over 
the eyeball to be attached obliquely to the sclera in the 
median line 7.7 mm. from the upper edge of the cornea. 
Its action is evidently not as simple as those of the internus 
and externum as can be seen from Fig. 83. When the eye 




is turned outward till its axis is directly in line with the 
course of the muscle (Fig. 83-5), the action would be 
simply that of turning the cornea upward in that same line. 
If the eye is looking directly forward (83-A), it not only 
elevates the cornea, but since the insertion is in front of the 
equator, it turns the cornea toward the nose at the same 
time. If the eye be already turned strongly toward the 
nose (83-C), its chief action is to rotate the eye wheel- 
fashion, tipping the top of the vertical plane inward and 
backward, an intorsion. In almost every position of the 
eye the action of the superior consists of varying combina- 
tions of elevation, adduction and intorsion. 



212 REFRACTION AND MOTILITY OF THE EYE. 

The inferior rectus passes from the common origin 
forward along the floor of the orbit slightly down and out, 
and is inserted 6.5 mm. from the lower edge of the cornea 
in the vertical plane. Its action is also a complex one. 
When the eye is directed outward (Fig. 83-5), so that the 
vertical plane is in line with the course of the muscle, it 
simply depresses the cornea and is a direct antagonist to 
the superior rectus. When the eye is directed straight 
forward (Fig. 83-A), however, it not only depresses the 
cornea, but adducts it, in which it assists the superior and 
internus. When the eye is strongly adducted (Fig. 83-0), 
it has a rotary action on the cornea, tipping the top of the 
vertical plane outward and forward, extorsion. The action 
of the inferior rectus then consists of varying combinations 
of depression, adduction and extorsion. 

The superior oblique rises from the common origin and 
passes along the wall of the orbit to the trochlea or pulley 
just inside the upper inner margin. The trochlea consists 
of a firm fibrous loop, in which the tendon of the muscle 
can slide back and forward and from this the tendon bends 
backward and outward at an acute angle and is inserted 
obliquely in the mid-line of the sclera, but behind the 
equator. We shall understand its action better, if we 
imagine it arising from the trochlea, since that is the 
direction of its traction. If the eye be turned strongly in, 
so that the vertical plane is in line with the course of the 
muscle, since it is inserted behind the equator in the 
posterior half of the eye, its action is simply that of a 
depressor of the cornea (Fig. 83-0). When the eye is 
directed straight forward, by virtue of its posterior inser- 
tion, it not only depresses the eye somewhat, but also 
abducts it and intorts it (Fig. 83-A). From its insertion 
its chief function is torsion (Fig. 83-2?). 



BINOCULAR VISION. 213 

The inferior oblique rises from the inner lower margin 
of the orbit, passes outward, backward and downward and 
is inserted near the mid-line of the sclera, but behind the 
equator. Its action will evidently be chiefly that of extor- 
sion, while, when the eye is turned strongly in, it will be 
an elevator, and when the eye is turned out, it will combine 
torsion with abduction and elevation (Fig. 83). 

Exactly what part each muscle plays in the coordinated 
movements of the eyes we do not know; we can only 
theorize. The simple movements, such as adduction and 
convergence, can be occasioned by the action of the interni 
alone. A combined action of the superior and inferior 
recti would aid convergence, but we cannot say that they 
are so used regularly or even in case the internus alone is 
insufficient. Xeither can we say they are not so used. 

Abduction of the eye can be accomplished by the 
externus alone. It would be aided, theoretically, by the 
combined action of the two obliques, but we have no proof 
that it is or is not. The more complicated motions are far 
beyond us as yet and are among the complicated problems 
of physiology. Still further, the motion of the eye con- 
sists not only of an active contraction of one muscle, but a 
limiting contraction of its opponent or opponents. Other- 
wise the eyeball would turn as freely as a turnstile and 
continually overshoot the mark. But all the various move- 
ments are dominated in health by the imperious necessity 
of -ingle binocular vision. The selection of this muscle 
or combination of muscles, the extent of the contraction of 
each and their coordination are all so determined by the 
brain that the two images of the object of regard shall fall 
on corresponding parts of the two retinae. A moment's 
thought will show how in the simplest movements the most 
delicate picking and choosing of muscles is necessary. If 



214 REFRACTION AND MOTILITY OF THE EYE. 

we turn the eyes to some object on the right, we contract 
the right externus and left internus, but not only must 
their contraction be exactly equalized to prevent diplopia, 
but their antagonists, the left externus and right internus, 
must stop the movement at exactly the proper time by a 
coordinative action, lest the eyes overshoot entirely the 
object of regard. 

The chief function of the oblique muscles is to keep 
the vertical planes of the two eyes always parallel by an- 
tagonizing the torsion which we have seen is inseparable 
from the action of the straight muscles in certain positions. 
It is probable too that in certain positions of the eyes they 
reinforce the superior and inferior recti and act as elevators 
and depressors. For instance when both eyes are turned 
to the right and up, the right superior rectus is a pure 
elevator and acts more efficiently than the left which is 
in a position to cause torsion instead of elevation. The 
left inferior oblique would here act as an elevator and re- 
inforce the superior rectus. In other positions of the eyes 
there is always an oblique muscle so placed as to assist 
the straight muscle when it is at a mechanical disadvantage. 

The Nerve Supply of the Muscles. — The move- 
ments of the eyes are regulated by centres of different rank. 
In the first place there are centres which govern the action 
of each individual muscle, the nuclei of the nerves which 
supply each one, electrical stimulation of which would 
result in uncoordinated movements of the eyes. These are 
situated on the floor of the fourth ventricle and have been 
pretty well localized by experiments on the brains of 
monkeys. The most anterior one is the nucleus of the 
motor oculi, or third nerve, which consists of several pairs 
and one unpaired group of ganglion cells and in a physio- 
logical sense must be regarded as being composed of a 



BINOCULAR VISION. 215 

number of nuclei, each of which innervates a separate 
muscle, though the exact localization in man has not yet 
been worked out. The fibres from these associated nuclei 
unite in a common trunk at the base of the brain which 
passes forward to the orbit and supplies all the ocular 
muscles except the externus and superior oblique, giving off 
filaments to the ciliary muscle and iris. 

The trochlear nerve which supplies the superior oblique 
has its nucleus right behind that of the third nerve, of 
which it might almost be regarded as a partial nucleus. 
The abducens which supplies the external rectus has the 
nucleus still further back on the floor of the fourth ven- 
tricle. It will be noted that the muscles which act together 
have their nerve nuclei together, as for instance convergence 
is always accompanied automatically by accommodation 
and contraction of the pupil. In the same way the nucleus 
of the superior rectus is near that of the inferior oblique 
which is supposed to assist it in elevating the eye, while the 
superior oblique and inferior rectus have their nuclei in 
propinquity. 

Presiding over these are centres of higher rank for 
coordinating the action of the muscles. Their location is 
unknown. They depend for their stimulation on the visual 
sensation made by objects on the retina and govern the 
muscles by acting through the nuclei of the nerves already 
alluded to. Their function is the preservation of binocular 
vision. Their action is entirely beyond our control except 
that we can affect them indirectly by manipulating the 
object of regard and so changing the visual stimulus. 

There is a third set of centres presiding over the ocular 
movements, namely the volitional ones by which we control 
the direction of our gaze. These centres also act through 
the nuclei in the fourth ventricle, but are subject to the 



216 REFRACTION AND MOTILITY OF THE EYE. 

second centre in that they cannot cause any conjugate 
action of the eyes which would result in diplopia. We 
can turn both eyes at will to the right or left, up or down, 
or combinations of these conjugate motions by aid of these 
voluntary centres which are located in the motor area of the 
cortex. If one eye be blind, or if we are in a dark room, 
or if we avoid fixing any object, the visual lines may or 
may not be parallel, but the instant a clear image is formed 
on both retinae, the involuntary coordination centres assert 
themselves in an automatic effort to avoid diplopia. 

The actual localization of all these centres has not been 
worked out, but they exist, since an injury to the motor area 
may entirely abolish the movement of both eyes to the right 
or left without paralyzing any ocular muscle. Or one may 
without any paralysis of his interni lose entirely his power 
to converge the eyes. Lastly we may have lesions affecting 
the basal centres which cause paralysis of one or more 
muscles and which, of course, interfere with both the 
voluntary and involuntary movements of the eyes. 

Binocular single vision is a very important function to 
the individual. An object seen with one eye singly appears 
flat, while seen by two eyes from slightly different points of 
view it gives the impression of depth and solidity. With a 
single eye judgment of the size of objects depends on the 
size of their image on the retina. In such a case a small 
object near by and a large one further off appear to be of 
the same size. With both eyes functioning our judgment 
is aided not only by the size of the respective images on the 
retinas, but by their distance as estimated from the amount 
of convergence necessary in fixing each. A one-eyed in- 
dividual can learn to tell whether a surface is raised or 
depressed by interpretations of light and shade, and to 
estimate fairly distance and size. 






BINOCULAR VISION. 217 

Binocular single vision occurs when the images of the 
object of regard fall on identical portions of the retinae and 
is converted with binocular double vision whenever one of 
the two eyes leaves the correct position of fixation. A 
person who is blind in one eye or who has learned to sup- 
press one image sees singly, but this is not binocular single 
vision. 

We direct a patient to look at a candle flame at a 
distance with both eyes. If one eye is manifestly turned 
in or out, binocular vision cannot be present, and he must 
either be blind in the deviating eye or be suppressing its 
image or seeing double. If we now cover its fellow, the 
deviating eye will, unless blind, move so as to bring the 
image on the macula and the slightest movement of redress 
is evidence not only of sight, but that, if vision is single, 
it was kept so by suppressing one image. The movement 
of redress can be distinctly seen in cases where the 
deviation is too slight to be apparent. There are also cases 
in which the eyes appear to deviate without actually doing 
so. In these there will be no movement of redress. 

As we shall see a little later, if two images are seen, 
their relative position, distinctness and distance apart are 
very valuable means of determining the deviation of the 
visual axis. A similar test near at hand can be made at 
a distance of ten inches with a pencil and a card, the 
observer standing directly in front of the patient whose 
eyes should be in a good light. 

Another test of binocular vision is this. If we place 
over the right eye a prism of 6° base down, the object 
flame is displaced upward and the patient sees two flames, 
one above the other. If, however, he sees only one it is 
because he suppresses the other or is blind. 

If a patient whose eyes are convergent looks at a 



218 REFRACTION AND MOTILITY OF THE EYE. 

distant candle, he will see two images, one of which is 
formed — let us suppose — at the macula of the right eye 
and is distinct and! correctly located. The left eye has 
rotated so that the image falls on the inner half of the 
retina and, being extramacular, is indistinct and hence 
called the false image. But the patient long ago learned 
that objects whose images fall on the right half of either 
retina, are actually on the left side of the body ; conse- 
quently, while the true image seems straight in front, the 
false one seems to be farther to the left. If the left eye 

was unduly divergent, 
^T >v the image would fall 

y~^ — _^^^ ^ on the temporal half 

"""*"■ """~^=»* aru ^ seem t° De to the 

/ ^N^ — — ' right of the real im- 

r ~y age. If the eye were 

^ — ^ n . depressed, the false 

FIG - 84 - • u , , -, 

image would be formed 

on the lower half and would seem to be above the true 
image. When the false image is situated on the same side 
of the body as the eye to which it belongs, the diplopia is 
said to be homonymous and always indicates convergence of 
the visual lines. When it is on the opposite side it is said 
to be crossed, invariably indicating divergence. When one 
image is higher than the other, the diplopia is vertical. 

Vertical diplopias are further distinguished by noting 
the eye which has the lower image. If the image formed 
in the right eye is lower, it is right diplopia, and vice versa. 
We can discover to which eye each image corresponds by 
covering one and having the patient tell which image dis- 
appears, or by placing a red glass before one eye so that the 
image formed in the eye shall be red. 



CHAPTER XI. 

NOKMAL MOTILITY. 

In examining the motility of the eyes there are three 
conditions to be investigated: first, the relation of the 
visual axes to each other, when the muscles are completely 
relaxed — the position of rest; second, the ability to keep 
these axes so related to each other in the ordinary move- 
ments of the eyes that binocular single vision shall always 
be present — the fusion power; and third, the voluntary 




t 
t 



Fig. 85. 

power to move the eyes as a pair in various directions — the 
power of rotation. Each one of these requires its own 
special tests and has its own special significance under 
various conditions. 

The Position of Eest. — We remember it was the 
chief function of the coordinating centres controlling the 
fusion power to keep the optic axes parallel for distant 
objects and to avoid diplopia, and we can only determine 
whether that is their natural position or not by abolishing 
for the time being the power of fusion, which may be done 
in several ways. 

Prism Test. — We use as our test object the flame of a 
candle or some distinct round object of equal size placed at 
a distance of twenty feet (Fig. 85). If we now place over 
the patient's right eye a prism of 5 A base down, the image 

(219) 



220 REFRACTION AND MOTILITY OF THE EYE. 



of the flame is thrown below the macula of that eye and the 
candle appears displaced upward. If the prism were a 
weaker one, the eye would, under stimulation of fusion, 
rotate in such a way as to bring the macula to the new site 
of the image, but if the prism is strong enough the/ fusion 
power is overcome and the patient sees two flames. If one 
flame is directly above the other, it is evident that the 
vertical planes of the eyes are still parallel. If the eyes 
were turned in or out, the candles would not only be dis- 




Fig. 85a. 



placed vertically, but also laterally. Kemoving the first, 
we now take a prism of 8 A or 10 A? or one strong 
enough to produce diplopia and place it over the eye with 
the base in, the flame is displaced toward the apex, and if 
the patient sees two flames side by side, the axes must both 
be in the horizontal plane, since, if the eyes deviated 
vertically, one candle would be higher than the other. 
A convenient application of this test is made with the 
Stevens phorometer (Fig. 85a) consisting essentially of 
two 5 A prisms, one before each eye, and rotating in 
opposite directions, giving the same displacement as a 10 A 
prism. With the prisms rotated base in, in such a way 
that two lights appear exactly on the same level the amount 



NORMAL- MOTILITY. 



221 



of hyperphoria is indicated by a pointer. With the prisms 
rotated so that one light appears on a line above the other 
the amount of esophoria or exophoria is similarly indicated. 
This is a very delicate test of the relative position of the 
visual lines to each other, and there are a number of tests 
based on similar ideas. If, for instance, we place a red 
glass over one eye, it reduces the illumination and causes 
the flame to appear red. The tendency to fuse a red flame 
and a white one is much less than two white ones, and if 





Fig. 

the patient sees them side by side, the optic axes must be 
deviating laterally, while if one is above the other, they 
must be deviating vertically. The fusion power is, how- 
ever, not entirely abolished, and the test is nothing like as 
delicate as the preceding. The advantage from a practical 
point of view is that the patient can describe the relation of 
the images more accurately by their color, and that when 
diplopia is present the lateral and vertical deviations are 
shown at the same time. Absence of diplopia does not 
show a normal balance. 

Maddox Rod (Fig. 86) consists of a glass rod or series 
of rods closely touching each other so as to produce the 
optical effect of a very powerful convex cylinder. This is 



222 REFRACTION AND MOTILITY OF THE EYE. 

placed in a trial frame before the right eye when the 
flame of the candle at once appears as a long, narrow band 
of light at right angles to the axis of the rods. There is no 
tendency to fuse two images as dissimilar as a candle flame 
and a band of light, hence each eye assumes the position of 
rest. When the band of light is vertical, it should pass 
directly through the flame as seen by the other eye, and any 
separation shows a lateral deviation inward or outward. 
The rod is now turned in the trial frame till the band is 
horizontal, when it should still pas© through the flame, any 
separation in this position showing that in the position of 



£ 



Fig. 86a. 

rest one eye points higher than the other. This is a very 
good test. It is to be noted, however, that in the great 
majority of patients, when the band is vertical the test 
shows a slight convergence of the optic axes which other 
tests often do not show (Fig. 86a). 

Duane's Test. — Using a distant candle and placing 
ourselves in such a position that we can see both eyes dis- 
tinctly we interpose a card or screen, first in front of one 
eye and then the other. The eye behind the screen, no 
longer being stimulated by the image of the candle flame, 
at once assumes the position of rest. If the visual lines in 
this position are divergent we can see the eye turn out be- 
hind the screen and if we then withdraw the card the eye, 
in again fixing the flame, makes a distinct movement of 



NORMAL MOTILITY. 223 

redress inward. If the eye deviates in behind the screen, 
with a movement of redress outward when the card is 
removed, we know that the position of rest must be one of 
convergence and the same reasoning holds good of vertical 
deviations, the movements of deviation and redress being 
up and down; while if no motion of the eye can be de- 
tected either behind the card or when the card is with- 
drawn, the visual lines are practically parallel and the 
position of rest nearly normal. If the eyes deviate out- 
ward behind the card we can, by placing a suitable prism 
base in over the eye, so change the direction of the rays 
from the flame that on the withdrawal of the card they 
fall on the macula without the necessity of any movement 
of redress ; while if the prism is too strong the movement 
will be reversed, the eye turning outward instead of inward 
when the card is withdrawn. Duane, who suggested this 
test, considers that if a distinct reversal of the movement 
of redress occurs with a 10° prism there will be no per- 
ceptible movement in either direction with a 7°, 8° or 9°, 
and that the 8° is therefore very nearly the mean deviation. 

Position of the Vertical Planes. — Using the candle 
flame as an object as before, we place before the one eye a 
Maddox rod with its axis carefully horizontal and over the 
other a similar rod with' axis vertical. From the first the 
eye gets the impression of a vertical band of light, while the 
other eye sees a horizontal one. If the vertical planes of 
the two eyes are exactly parallel in the position of rest, these 
bands should intersect at right angles. If the vertical 
planes are not parallel, the angles of intersection will not 
be right angles, and one eye or both must evidently have 
been rotated wheel-fashion, either intorsion or extorsion. 

Double Prism Test. — The Maddox prism is composed 
of two prisms in a trial ring, so arranged that their bases 



224 REFRACTION AND MOTILITY OF THE EYE. 

touch, their line of junction passing exactly through the 
centre of the ring. If this is placed before one eye, so that 
this line passes horizontally in front of the centre of the 
pupil, it will cause a monocular diplopia, and the horizontal 
line used as a test object will appear as two parallel lines. 
If, now, the other eye be uncovered, it sees a single line 
which, if no torsion of either eye has taken place, should be 
between and parallel to the other two lines. 




Fig. 87. 



Convergence and Accommodation — Metre Angle. — If, 
in the position of rest, the optic axes are parallel, a point 
of light infinitely distant will be seen singly without 
either convergence or divergence. If, now, this point be 
approached to a distance of one metre, the eyes move 
through a definite angle known as a metre angle — (M. A.). 
Evidently if the object be two metres distant, the angle 
will be only .5 M. A., while if the object is still seen singly 
at half a metre, the convergence amounts to 2 M. A. 

Attention has before been called to the fact that there 
is a normal relation between convergence and accommoda- 
tion. In the previous test, for instance, in fixing an object 
at a metre, the patient not only converges' 1 M. A., but if 



NORMAL MOTILITY. 



225 



his refraction is normal, accommodates 1 D. ; at a half 
metre 2 M. A. and '2 D., etc. This intimate relationship 
is also indicated by the close association of the centres for 
convergence and accommodation. So close is this relation- 
ship that, if, owing to refractive conditions, accommoda- 
tion is increased or reduced, the tendency to converge 
undergoes a corresponding change which has to be com- 
pensated for in some way. It follows that just as in the 
primary position of the eyes there should be single vision 
of distant objects without the stimulation of fusion, so for 
near objects there is a state of equilibrium between accom- 




Fig. 88. 



modation and convergence, so that the optic axes auto- 
matically fix the same point without the necessity of fusion 
stimulation. To discover whether the eyes are in a posi- 
tion of equilibrium for near work, the following tests are 
useful. 

Cover Test or Screen Test. — Having the patient fix 
the point of a pencil or other small object at a distance of 
eighteen inches, we interpose before one eye a card. If the 
balance between accommodation and convergence is perfect, 
the covered eye automatically maintains its exact position. 
If convergence is in excess the covered eye having nothing 
to fix, turns in, while if it is insufficient it turns out, and if 
these deviations are extreme they are apparent when the 
observer looks behind the screen, but it is often too slight to 

15 



226 REFRACTION AND MOTILITY OF THE EYE. 

be made out in this way. If, however, we closely watch 
the screened eye and quietly withdraw the screen, it will at 
once fix the object by a movement of redress. If it moves 
in, it must have been divergent behind the screen, while if it 
moves outward the convergence must have been in excess of 
the accommodation. 

Graefe Equilibrium Test. — If we place before the right 
eye a prism of 5 A* b&se up, and have the patient look at a 
black point on a card held at the ordinary reading distance, 
the image seen by the right eye will be displaced and ap- 



5 4 3 2 1 123 45 

iJ III 1^1 III I 



Fig. 88a. 

pear lower down than its fellow. But if both eyes accom- 
modate for this distance, they should also automatically 
converge for the same distance and one dot should be 
exactly above the other, and any lateral deviation shows an 
abnormal proportion which must be rectified by stimulation 
of the fusion power. This can be measured by the prism 
required to bring the dots into line, or a card used show- 
ing on its printed scale the prism equivalent of the dis- 
placement. 

Fusion Power and its Measurement.— It is evident 
that diplopia for distance would occur in all eyes whose 
position of rest was not with axes parallel. Diplopia at the 



NORMAL MOTILITY. 227 

near point would regularly occur whenever the eyes were 
not in a position of equilibrium at that point. In other 
words, even- conceivable lack of balance of the extrinsic 
ocular muscles and every refractive error would produce 
more or less diplopia, unless nature had provided some 
means of compensating for her defects. This compensa- 
tion is provided by the fusion power which is presumably 
in abeyance in normally balanced eyes, but which, in the 
presence of diplopia, imparts an extra stimulation to the 
muscles necessary to correct it. 

While the determination of the position of rest and 
equilibrium depends on our power to abolish fusion, the 




Fig. 89. 

coming series of tests of the compensatory power of the 
eve depends on its highest stimulation. No matter how 
good the condition of the muscles, fusion will not occur if 
the retinal stimulation be so slight that the image in one 
eye can be easily suppressed. To bring out the utmost 
fusion power the vision in each eye must be made as perfect 
as possible. 

Convergence Tests. — The patient regards the flame of 
a candle at a distance of twenty feet and a prism of 5 A 
is placed before the left eye with the base out. The image 
of the flame, instead of falling on the macula, is deflected 
toward the base of the prism and falls on the outer half of 
the retina causing a cross diplopia. Under the stimulus of 
fusion the macula is immediately rotated outward to the 



228 REFRACTION AND MOTILITY OF THE EYE. 

new site of the image by a contraction of the internal 
rectus and the images are foeussed and the diplopia 
vanishes. We now take stronger and stronger prisms in 
turn, each one increasing the distance of the image from 
the macula and consequently the contraction of the internus 
necessary to secure single vision. Finally we reach one so 
strong that the internus has not the required strength to 
overcome it and the diplopia persists. The strongest prism 
which has been successfully overcome measures the ability 
to converge that eye. 

The value of the test depends largely on the care and 
skill with which it is made. Since it depends on the utmost 
stimulation of the fusion power, the value is impaired by 
anything which reduces the clearness of vision in either eye. 
Consequently, it is a mistake to follow the common plan of 
determining the presence or absence of diplopia with the 
red glass and then, without removing this, to measure the 
prism convergence. The image seen through the red glass 
is not only much fainter, but of a different color, and only 
a slight effort is made to fuse images so dissimilar. Plenty 
of time should be allowed before deciding that the limit of 
strength has been reached, for very frequently after stimu- 
lation by a few weak prisms the eyes will overcome a prism 
diplopia which would have been hopeless at first. Many 
consider the ability to overcome prisms more of a knack 
than a measure of actual strength. 

This leads to another disadvantage of the test as 
ordinarily made. The sharper the vision, the greater the 
fusion tendency. Consequently, weak prisms which dis- 
place the image but a short distance from the macula are 
readily overcome, but when strong ones are reached which 
throw the image to the periphery of the retina, not only are 
the images very far apart, but one of them is very indis*- 



NORMAL MOTILITY. 229 

tinct and the stimulation of fusion is very much reduced. 
To overcome this defect, the 

Rotary Prisms have been devised, consisting of two or 
more prisms rotating in opposite directions by a screw in a 
common frame and thus furnishing a prism which can be 
varied from to 30 A- With such a prism, gradually 
increasing in strength without being taken from the eye, 
the image is always just beyond the macula where the 
fusion power is strongest and the eye thus stimulated to 
its extreme power. Great care 
should be taken in estimating 
the convergence with prisms, 
that the patient does not twist 
or tilt the head and that the 
prism has its axis so that the 
images are exactly horizontal, 
since in this position the eyes 
will overcome much stronger 
prisms than when one is higher 
up than the other. 

If now the other eye be tested in the same manner, we 
shall ordinarily find that the strength of its internus is 
practically the same. We might at first sight assume that 
if each eye can overcome a prism of a certain strength, the 
two together ought to overcome the combined prisms. It 
will be found that the two together can accomplish little 
more than either singly. While the internus of the eye 
behind the prism is in a state of contraction great enough 
to move the eye, the internus of the fellow eye is almost as 
tense to preserve its own position. Differences) in the 
prism power of the two in tern i are ordinarily due to differ- 
ences in the amount of fusion stimulation. If one eye has 
reduced vision it may hold its position very well while the 




230 REFRACTION AND MOTILITY OF THE EYE. 

image is formed at its macula and so allow the better eye 
to overcome a strong prism. When, however, the prism is 
transferred to the worse eye and the image thrown away 
from the macula, the retinal stimulation is so feeble that 
the true prism power is not shown and very often the image, 
if faint, is suppressed altogether. 

Divergence Tests. — We place over one eye a weak prism, 
say 2 A? with its base in, carefully adjusting its axis 
horizontally. The rays from the candle are so deflected 
by the prism that they fall to the inner side of the macula, 
and being projected in the direction of the apex of the 
prism, cause a homonymous diplopia. Under the fusion 
stimulus, the external rectus contracts so as to rotate the 
macula inward to the site of the image and abolish the 
diplopia. We then gradually increase the strength of the 
prism and the strongest one which the extemus can over- 
come is the measure of the divergence power. If we 
proceed to apply the same test at once to the other eye, we 
shall probably find that its power is apparently somewhat 
less, owing to this fact : the two externi work together in 
divergence, the one behind the prism being active and the 
other passive, so that the second is already tired before 
being measured. We have seen that the convergence power 
seems to increase through the exercise of testing, because 
convergence is an act to be actually performed through 
powerful muscles. Divergence is not called for in normal 
eyes and is performed through relatively weak muscles, and 
prism exercise does not increase the divergence power 
materially. 

Sursumvergence. — If we place before one eye a weak 
prism, say 2 A, with the base down, the rays from the 
candle fall below the macula and the image in that eye 
being projected appears directly over that of the other a 



NORMAL MOTILITY. 231 

vertical diplopia. This is corrected by the superior rectus 
which, by contracting moves the macula down to the other 
site of the image. Gradually increasing the strength of the 
prism, the strongest which can be overcome is the measure 
of the strength of the superior rectus. 

Deorsumvergence. — The strongest prism, base up, 
which can be overcome is in a like manner the measure of 
the power of the inferior rectus. 

In these last tests it must not be forgotten that, while 
the superior of one eye is actively contracting to overcome 
a prism, the inferior rectus of the other is nearly as tense 
to prevent its also moving upward, so that tests of the 
vertically acting muscles involve both eyes. 

Torsion. — It must also be borne in mind that the 
oblique muscles may take part in the elevating and depress- 
ing of the eyes and, if so, their power is included in these 
tests. Their chief function, however, is in maintaining a 
parallel position of the vertical planes of the two eyes. We 
have already seen how to ascertain whether they are 
parallel in the position of rest. We now place before each 
eye a Maddox rod, axis horizontal, which causes the patient 
to see two vertical bands of light which, being exactly 
alike, are immediately fused. If now we rotate one rod 
slightly, the band ceases to be vertical, but under the 
fusion stimulus the eye rotates wheel-fashion, so that the 
band shall still fall on the vertical plane of the retina. 
When the rod is rotated further, beyond the power of the 
oblique, the patient sees two bands crossing each other at 
an acute angle. The number of degrees through which the 
rod can be rotated without producing two bands, is a rough 
measure of the ability of the oblique muscles to keep the 
vertical planes parallel. It is not to be forgotten that the 
oblique muscles work in pairs and we have no means of 



232 REFRACTION AND MOTILITY OF THE EYE. 

estimating each separately. Stevens has devised a very 
ingenious instrument, the Clinoscope, for measuring and 




recording the torsion powers of the obliques. The instru- 
ment consists of two cylindrical tubes mounted on a brass 
platform, which holds them firmly in the same horizontal 



NORMAL MOTILITY. 233 

plane. The attachment to the platform permits the tubes 
to be adjusted in parallelism, in convergence, or in 
divergence in the plane of the platform. The platform is 
attached by a movable joint to the upright standard, so that 
the instrument may be given any desired dip, and a scale 
and pointer indicate the dip with respect to the horizon. 
The tubes are caused to rotate upon their longitudinal axes 
by means of thumb screws, as seen in the figure, and the 
pointer and scale above the tubes mark the rotation with 
accuracy. At the proximal end of each tube is a clip, in 
which the observer may insert a glass for the correction of 
refraction. At the distal end is another clip and provision 
for maintaining precise position of the diagrams to be used 
in the investigation. These diagrams are haloscopic 
figures, calculated to aid in the various experiments which 
may be made. These may be varied according to the wish 
of the investigator. For testing the ability of the eyes to 
rotate upon the antero-posterior axis (torsion), a straight 
line running across each disc is the most useful figure. 
The lines may be placed vertically or horizontally. It will 
be found that the rotating ability is much greater when the 
lines are vertical. 

The clinoscope is an instrument of much value in 
determining the declination of the meridians in paralysis 
of the eye muscles, in anomalous adjustments of the eyes in 
respect to the horizontal visual plane and in determining 
the power of torsion or increasing the torsional ability by 
exercise. 

Relation of Fusion Powers. — So far nothing has 
been said to indicate the amount of fusion power which the 
normal eye should have, because there is no fixed scale to 
which all agree. Fusion depends first on retinal sensation; 
second, on the amount of reflex nerve stimulation, and, 



234 REFRACTION AND MOTILITY OF THE EYE. 

lastly, on the size and strength of the muscles. In all 
these particulars individuals vary widely. But if there is 
no exact standard of measurement which can be applied to 
different individuals, there is a fairly definite proportion 
which subsists between the fusion power of different 
muscles in the same individual, any marked departure from 
which is pathological. 

Convergence, for instance, is a power which is con- 
tinually used by all of us and yet, while the average 
individual can overcome at the first trial a prism of 15 A 
to 20 A? there are many who can with difficulty overcome 
10 A and yet have no symptoms, while occasionally 
another can develop 40 or 50 A- Divergence is not so 
useful and is always weak, varying from 2 A up to 12 A> 
but if the same individual has a convergence of 40 A 
and a divergence of only 2 A? it is very apt to occasion 
symptoms which we shall take up later. Ordinarily, 
divergence should be about 5 A or 6 A and convergence be- 
tween 15 A and 20 A? or in this proportion. In other 
words, convergence should be from three to four times as 
strong as divergence. 

The ability of the inferior is always greater than the 
superior, the latter averaging 2 A to 4 A and the former 
3 A to 6 A- The proportion between superior and inferior 
is not so important, so long as it is the same in both eyes, 
since as long as the visual lines are in the same horizontal 
plane, that plane can be raised or lowered by tipping the 
head up or down without moving the eyes in the orbits. 

If the power to overcome prisms is weak but pro- 
portionate, it may be because of poor vision, retinal 
anesthesia or failure of the fusion centres. If it is very 
great and proportionate and well maintained, it indicates 
strong stimulation and centres and healthy muscles. If 



NORMAL MOTILITY. 235 

not maintained, it shows innervation in excess of muscular 
power. If not proportionate, since the fusion impulse is 
equal for all, it indicates an abnormality of muscular 
balance. 

We have studied the fusion power which is a matter 
chiefly of innervation and not of actual muscular power. 
For instance, in converging the right eye to overcome a 
prism of 40 &, the internus only moves the cornea 
through an arc of 20° of a circle, and in diverging 5° the 
cornea only moves 2.5°, while it is evident that in turning 
both eyes to the left, the internus moves the cornea many 
degrees farther, and in turning both eyes to the right, the 
externus has almost as great a power. Evidently there 
must be a motility of the eyes we have not measured as yet, 
and that is the muscular power when under the control of 
the will. 

Voluntary Motioxs of the Eye. — These consist of 
movements of both eyes together to one side or the other or 
up or down or combinations of these movements. When 
gazing at objects directly in front, the fusion instinct keeps 
the axes parallel and so coordinates the motion of the two 
eyes, but in extreme rotations to the right or left or up or 
down, a screen is interposed over one eye in the shape of 
the nose or cheek or brow so that binocular vision is 
impossible and the extreme rotation occurs without the 
usual limitation interposed by the necessity of binocular 
vision. Since the rotation of the eye in these conjugate 
movements are much greater than in any other, since they 
can be made of each eye singly without regard to the other 
and since they are voluntary, their movement is the truest 
test of the individual muscles of the eyeballs. 

Linear Measurement. — This is a very rough test which 
will detect only gross variations from the normal and can 



236 REFRACTION AND MOTILITY OF THE EYE. 

be applied only to the inward and outward rotations. 
Having the patient place his eyes in the primary position, 
we direct him to look to the right as far as he can and then 
to the left, noting whether the margin of the cornea passes 
or falls short of the respective canthi. Or a mark can be 
placed on the lower lid showing the position of each margin 
of the cornea. But such tests are absolutely unreliable 
where accuracy is desired. 

Measurement by the Perimeter. — We place the patient 
before the instrument with the eye to be tested carefully 
adjusted to the sight notch. As our test object we take a 
small card with several small dots at the centre, and 
beginning at the centre of the arc, slowly move it outward, 
the patient being directed to follow with the eye but not 
with the head. After the card has reached 45° more or 
less, the patient has reached the limit of ability to move 
the eye, and as he can no longer keep the macula on the 
dots he ceases to see them clearly and separately. Making 
a note of the reading on the arc opposite the centre of the 
card, we proceed to measure the inward rotation in the 
same way, and then placing the arc in a vertical position we 
measure in like way the upward and downward rotation. 
It often happens, however, that one has to measure the 
excursions of eyes which have poor sight either intrinsically 
or without glasses, where the dot test is unavailable, and 
there is always the difficulty of getting the patient to dis- 
tinguish between mere seeing and the distinct vision which 
indicates the position of the macula. We can make the 
test more objective by using a candle or small electric light 
instead of the card. If the patient be directed to fix the 
light at the centre of the arc, the observer, whose eyes 
should be directly behind the light, will see a distinct 



NORMAL MOTILITY. 237 

reflection in the centre of the patient's pupil. Directing 
the patient to follow, and keeping his own eye in line, the 
observer moves the light along the arc till the reflection 
ceases to be seen in the centre of the pupil, which indicates 
that the eye has reached the limit of its ability to follow. 
The mark on the arc opposite the light indicates the ability 




Fig. 92. 

of the eye to rotate in that direction. In this way we can 
get a fairly accurate estimate of the contraction of each 
muscle, the outward rotation being caused by the externus 
alone averaging about 45°, the inward by the internus alone 
averaging a little higher, about 50°. The downward 
rotation is caused theoretically by the combined action of 
the inferior rectus and superior oblique and averages 40°. 
If we wish the action of the inferior alone, we can adjust 



238 REFRACTION AND MOTILITY OF THE EYE. 

the head so that when the eye fixes the centre of the arc, 
it is turned slightly outward in such a position that the 
oblique loses its depressing power and effects only torsion. 
The upward rotation caused by the superior rectus and 
inferior obliques measures about 30°, partly because these 
muscles are weaker and partly because the position of rest 
of the eye is slightly below the horizontal. By turning the 
eye slightly outward, we can nullify the elevating action of 
the oblique and get the power of the superior alone if we 
wish. 

There are some disadvantages about the perimeter 
method, chief of which is this, that in a patient with a 
prominent nose the entire inward rotation cannot be 
measured, since the bridge of the nose prevents alike the 
patient seeing the light and the observer, keeping in line 
with the light, from seeing the cornea. With a deep orbit 
or prominent brow and cheek the upward and downward 
rotations are obtained with difficulty. The perimeter also 
fails of any method of placing and keeping the patient's 
head in a symmetrical position. Evidently, if the patient's 
head is tilted forward, he will have a much reduced ability 
to elevate his eyes and a corresponding increase in his 
downward rotation. If his head be turned slightly to the 
right, his eyes will be turned toward the left, and he will 
have a correspondingly diminished rotation toward the left 
and increased toward the right. The tendency too is for 
the patient in his eagerness to follow the light to turn his 
head when his eyes have reached their limit. Therefore, 
the perimeter, while in theory an exact instrument, is in 
practice liable to cause many errors which can be avoided 
only by experience and skill. 

The Tropometer of Stevens retains all the advantages 
of the perimeter and does away with most of its dis- 



NORMAL MOTILITY. 



239 



advantages. It provides means for ascertaining that the 
head is erect and neither turned to the right nor left and 
that the eyes are on the same level; in this position the 
head is secured by gripping a bar of wood with the teeth and 
t by the pressure of clamps on the forehead and occiput. 
The movements of the eye are observed through a telescope, 




Fig. 93. 



the corneal margin traveling along a scale corresponding to 
the degrees on the perimeter. 

With this instrument it is possible to measure not only 
the rotation of the eye up and down, in and out, but by 
adjusting the head so that the oblique muscles lose their 
elevating and depressing power, we can estimate directly 
the power of the superior and inferior recti. 

We have seen that in the ideal patient the two eyes 
should not only be absolutely emmetropic, but 



240 REFRACTION AND MOTILITY OF THE EYE. 

1. In the position of rest their visual lines should be 
parallel, and their vertical planes as well; 

2. That the relation between convergence and accom- 
modation should be so perfect that in near work the visual 
lines should impinge on the same point without the inter- 
vention of any fusion impulse; 




Fig. 93a. 



3. That in conjoined movements to the right and left 
or up or down the eyes shall maintain single vision without 
the intervention of the fusion power. 

4. That where this ideal condition is not present, a 
working balance is maintained by the fusion power which 
depends on 

a. Eetinal stimulation, which can be assumed as 
normal when the visual acuity of each eye is normal ; 






NORMAL MOTILITY. 241 

b. Proper innervation, which is wanting when with 
good vision in each eye the patient cannot be made con- 
scious of diplopia by prisms and is normal when the 
proportion of prism power is normal; 

c. Muscular power — which is normal when the prism 
proportion is normal and the voluntary rotations of the 
eyeball normal in extent. 



19 



CHAPTER XII. 



HETEROPHORIA. 



Orthophoria. — The term orthophoria is used to denote 
an absolutely normal balance of the extrinsic muscles, just 
as the term emmetropia denotes a normal refractive con- 
dition. They are equally rare. 

Heterophoria. — The term heterophoria includes all 
those conditions in which there is a demonstrable tendency 
to depart from the normal balance but which nature is able 
to compensate for, while the term heterotopia includes the 
conditions in which nature has been unequal to the task 
and an actual turning or squint has occurred. 

The subdivisions of these terms at first sight appear 
complicated, but on closer study are simple enough, 
indicating only the direction of the turning or tendency to 
turn. For instance: — 



Esophoria signifies inward 
tendency. 

Exophoria signifies outward 
tendency. 

Hyperphoria signifies upward 
tendency. 

Hypophoria signifies down- 
ward tendency. 

Cyclophoria signifies tend- 
ency to torsion. 



Esotropia signifies inward 
turning. 

Exotropia signifies outward 
turning. 

Hypertropia signifies upward 
turning. 

Hypotropia signifies down- 
ward turning. 

Cyclotropia signifies actual 
torsion. 



Combinations are describable in similar terms. A 
tendency of the right eye up and inward is a right hyper- 
esophoria, of the left eye down and out a left hypo- 
(242) 



HETEROPHORIA. 243 

exophoria, etc. Tendencies of both eyes together are 
denoted by the terms which follow : — 

Anaphoria signifies an upward tendency. 
Kataphoria signifies a downward tendency. 
Dextrophoria signifies a right tendency. 
Laevophoria signifies a left tendency. 

The Cause of Heterophorias is a matter of dispute, 
some authorities claiming that they are invariably con- 
genital anatomical defects, and others that they develop as 
the result of uncorrected refractive errors. Probably the 
truth lies somewhere between the two extreme positions. 
The human body is never absolutely symmetrical and it is 
extremely improbable that any pair of eyes were ever 
exactly alike either in their refraction or their muscles. In 
this sense heterophorias are often congenital and primary. 
Occasionally we see also certain gross muscular defects that 
seem to be present in several generations of the same 
family and go with certain cranial types, but as a rule we 
have no evidence that children are often born with any but 
the slight defects of muscle balance which are easily and 
perfectly compensated for by the fusion powers. At 
present we have no means of estimating the muscle balance 
in young infants which entitles us to be dogmatic on either 
side. But granted a slight congenital imbalance, compen- 
sation through the fusion sense may be prevented by poor 
vision or defective centres, and the imbalance may increase 
indefinitely till it becomes an actual squint. Even if the 
balance was perfect at birth, refractive errors may grad- 
ually destroy it. For instance, hyperopia and astigmatism, 
which cause undue accommodation, at the same time cause 
undue convergence and thus produce an esophoria which is 
at first purely functional, but as the interni hypertrophy 
from overstimulation becomes anatomical. If this occurs 



244 REFRACTION AND MOTILITY OF THE EYE. 

after maturity, the hypertrophy is often a temporary one 
which gradually disappears when the cause is removed, but 
when it occurs in the plastic period of childhood, it is much 
more apt to result in a permanent increase in bulk and 
strength. If the muscle is stimulated beyond its powers of 
response, it finally atrophies and a divergence results. In 
this way can we best explain the divergence so common in 
the myope with his very close far point. 

Symptoms — These depend on the kind of error present 
as well as the degree, and vary widely. In general they 
may be said to fall into three classes, defective vision, pain 
of greater or less degree, and reflex symptoms. Defective 
vision may be present even though each eye has a normal 
visual acuity, since even when compensation is very good 
the brain gets the impression of two objects very nearly 
but not quite fused, and vision may be considerably worse 
with both eyes together than with either singly. When 
compensation is considerably impaired, the diplopia becomes 
more and more persistent, till the brain finally makes choice 
of one image as more satisfactory and suppresses the other 
entirely. Visual acuity may not suffer in either eye, but 
vision being no longer binocular, everything is seen in the 
flat, the judgments of depth and distance being regularly 
more or less defective. This is a tremendous disadvantage 
in many occupations. Patients gradually get accustomed 
to these visual defects and are not conscious of the handi- 
cap, but it is very different with the second set of 
symptoms — pains. 

These are more often present when compensation is 
maintained only by excessive effort. Such patients com- 
monly have good visual acuity in each eye and do not take 
kindly to diplopia, so the fusion centres transmit a very 
strong impulse to the extrinsic muscles to restore the 



HETEROPHORIA. 245 

balance. If the patient's general muscular condition is 
bad as the result of poor nutrition or disease, the eye 
muscles may respond, but soon tire, and continuous 
stimulation causes pain. In a minority of cases the 
muscles gradually strengthen from the increased innerva- 
tion and the pains disappear, compensation being fully 
restored. In many others the conditions remain unchanged 
for a long time, the eyes being fully competent for short 
periods of work, but regularly becoming painful when over- 
used. In another class where the muscular powers are 
weaker or the stimulation less, compensation breaks down 
completely with diplopia and suppression of one image, 
but with a great diminution of the amount of pain. 

The character of the subjective symptoms in refractive 
errors and muscular imbalance is so very similar that it is 
almost impossible to differentiate from these alone in many 
cases. In muscular asthenopia, however, in addition to 
becoming tired easily, the patient often complains that 
letters seem to run together or to "jump" while he looks at 
them, or that he sees double for an instant, or he can "feel 
his eyes turn" involuntarily in their sockets. Like accom- 
modative asthenopia, there are pains in the eyes and frontal 
headache, neuralgia, etc., but the characteristic pain in my 
experience is an occipital pain. These pains are some- 
times present only during use of the eyes. At other times 
they persist for hours afterward and, in some cases, at 
irregular intervals after days or weeks of overstimulation 
an explosion occurs lasting a day or two in the form of a 
migraine. 

In other cases there are other reflex symptoms such as 
dizziness, nausea, fainting, indigestion, insomnia and pains 
in other portions of the body which sometimes simulate 
organic diseases. 



246 REFRACTION AND MOTILITY OF THE EYE. 

The possibility of lieterophoria as a factor in chorea, 
epilepsy, migraine, neurasthenia and other diseases which 
may be primarily due to unstable nervous equilibrium, is 
not to be forgotten. It is a notable fact that when the 
fusion compensation fails so completely that one image is 
entirely suppressed or the diplopia is so great as to be 
overlooked, the symptoms often cease entirely. 

The Treatment of Heterophoria depends on a 
careful study of each individual case, but it can not be too 
strongly emphasized that in the great majority of cases the 
subjective symptoms disappear after a full correction of the 
refraction under atropin. 

In many cases we shall see that if the visual acuity in 
each eye be made normal, the fusion impulse alone will be 
sufficient to restore compensation. 

Many cases of esophoria result from overstimulation of 
the centres for convergence and accommodation made 
necessary by hyperopia and astigmatism, and disappear 
entirely when glasses abolish the need of accommodation. 
Cases of exophoria are sometimes due to the abnormal 
relaxation of accommodation and convergence which secures 
the best distant vision in myopia. Likewise the correction 
of myopia, by increasing the far point, may diminish the 
amount of convergence necessary for near vision. In every 
case the refraction should be carefully estimated under 
atropin and, as a rule, fully corrected. The advantages of 
slight overcorrection of hyperopia in a few cases of 
esophoria at the near point, with the idea of still further 
reducing convergence, and of slight overcorrection of 
myopia in exophoria with the idea of increasing the con- 
vergence stimulation and at the same time lessening the 
convergence necessity by removing the near point, should 
not be forgotten. 



HETEROPHORIA. 247 

Education of Fusion Impulse. — In a few cases, with 
perfect visual acuity in each eye, we can with difficulty 
make the patient conscious of diplopia and his fusion 
power is out of all proportion to his muscular power as 
revealed by the tropometer. This can be due only to 
defective fusion centres. In such cases, if we can once get 
the patient to be conscious of a diplopia, we can train him 
to overcome it and gradually increase his fusion powers. 
For this purpose the stereoscope or amblyoscope are very 
useful. This is not to be done thoughtlessly, for if restora- 
tion of binocular vision is to result in serious asthenopia, 
the patient is much better off without it. 

Improvement of Muscular Power. — In cases where the 
difficulty seems to be a general muscular weakness due to 
ill nutrition or disease, as evidenced by low but proportional 
prism power and nearly normal rotatory powers, much good 
can be done by general tonic treatment with rest from 
overwork. Especially is this true of muscular asthenopia 
developing after confinement or long illness in which a 
restoration to normal bodily condition often sees the dis- 
appearance of all asthenopic symptoms. 

Prism Exercise. — Where the tests point to a weakness 
of a pair of muscles as in deficiency of convergence powers, 
prism exercises have an undoubted value. The patient 
fixes a candle flame at a distance of twenty feet, having 
before one eye a rotary prism, base out; the strength of 
this is gradually increased till diplopia is produced, after 
which the strength is gradually decreased to the minimum 
and after a moment's rest the process is repeated. A few 
moment's exercise of each internus in this way several 
times a week, carried to the point of fatigue but never 
beyond, will often cause a tremendous increase in the con- 
vergence power from 7 or 8 A to 30 /\, but if the exercise 



248 REFRACTION AND MOTILITY OF THE EYE. 

is regularly carried beyond the physiological limit, it is 
harmful. In some cases the improvement persists, while 
in others it gradually disappears. 

If the asthenopia is due simply to weak interni, the 
externi being average according to the prisms and tropo- 
meter, the improvement is likely to be permanent. But if 
the poor convergence is due to overacting externi, prism 
exercises are of doubtful value. Exercise of the. other 
straight muscles is rarely beneficial. Divergence, whether 
vertical or horizontal, is, except within very narrow limits, 
not a physiological function, and the externi, the superior 
and inferior, do not increase their power notably through 
exercise. 

Prisms for Constant Use are often prescribed, being so 
placed as to help the weak muscles and counteract the 
strong. For instance, in exophoria we find the prism 
which, base in, will produce orthophoria for distance and 
prescribe a quarter of it, base in, before each eye. While 
this is very successful in some cases, the tendency 1 in others 



l We sometimes take advantage of this tendency when we 
prescribe for constant use weak prisms with the apex over the 
weak muscle, which gradually becomes strong from the exercise of 
overcoming them. This plan is effective only in patients who have 
a strong fusion impulse, and the prism selected must be weak 
enough to be easily overcome. We can accomplish the same effect 
by decentering the patient's refraction lenses. For instance, a 
convex lens so placed that the visual line passes to the nasal side 
of its optical centre will have the effect of a prism base out, and 
the reverse will be the case if the lens is concave. The amount of 
prismatic action depends on the strength of the lens and the 
amount of decentering, the rule being that every centimeter of 
displacement causes as many prism dioptres as there are dioptres 
in that meridian of the lens. Thus a + 1 sphere or cylinder 
axis 90 decentered one centimeter outward is equivalent to add- 
ing a 1 prism dioptre lens base out. 



HETEROPHORIA. 249 

is for the externus to increase slightly from constant 
exercise in overcoming the prism while the internus 
decreases in proportion to the amount of work of which it is 
relieved. This effect is still more marked when the prisms 
ar^ prescribed, base out. in esophoria. Prisms for per- 
manent use are very beneficial in vertical deviations, since, 
when the images are brought on the same level, they require 
much less effort to secure fusion, and when prescribed base 
up or down, the effect secured is commonly an unchanging 
one. 

Operative Measures should only be adopted after care- 
ful consideration and study of each case. They produce 
an effect only on individual muscles and have slight value 
in cases where heterophoria is the result of poor vision or 
reduced innervation. They are most useful where the 
tropometer shows an actual increase or decrease in the 
power of one muscle or pair of muscles as compared with 
its opponent. Three operations are at our disposal: 

Tenotomy, partial or complete, to weaken a strong 
muscle ; shortening, to strengthen a weak muscle ; resection, 
to strengthen a weak muscle. 

Partial tenotomy is intended to equalize muscular 
balance by weakening the power of the stronger. Theo- 
retically it should only be employed where prism tests and 
especially tropometer tests show that the muscle operated 
on is not only out of proportion to its antagonist, but also 
possesses more actual power to turn the eye than it should 
have. For instance a rotation of 45° in and an outward 
of 35°, would certainly not call for tenotomy of any sort, 
since it would still further reduce the power of an internus 
that is already hardly up to the standard. On the other 
hand, an inward rotation of 70° and outward of 45° would 
certainly call for a reduction of the power of the internus 



250 



REFRACTION AND MOTILITY OF THE EYE. 



rather than an increase of the externus and the tenotomy 
might be advisable. At the same time it must not be for- 
gotten that any operation which increases or diminishes 
the power of one muscle has a corresponding effect on its 
antagonist. For this reason partial tenotomy has been 
clinically beneficial in many cases in which it was not 
theoretically indicated. Its advantages are that it is a safe 
operation entailing a minimum of inconvenience to the 
patient, and that its effect can be tested from time to time 




Fig. 94. 



during the operation, if desired. Its disadvantage is that 
in many cases the process of healing and cicatricial con- 
traction destroys the effect of the operation, and that, if 
overdone, its rectification is not so simple. Under cocaine 
anesthesia and with or without a speculum the patient is 
directed to rotate his eye so as to bring the insertion of the 
muscle selected into the field. With a mouse-toothed for- 
ceps the conjunctiva, capsule of tenon, and tendon of the 
muscle are picked up together just back of the insertion 
and nicked with scissors if only a slight effect is desired, 
the line of incision being at right angles to the course of 
the muscle. The idea is to cut through the central fibre of 



HETEROPHORIA. 251 

the tendon which retracts, thus lengthening the muscle in 
proportion to the length of the incision. If it is desired 
to increase this effect, a small strabismus hook is slipped 
through the incision and on this the incision in the muscle 
is lengthened without further cutting the conjunctiva. No 
suture of the conjunctiva is necessary and no bandage. 

If preferred, a triangular flap of conjunctiva may be 
laid back, a small incision made through the capsule of 
Tenon either above or below so that a small hook can be 
passed under the entire muscle. In this position one can 
divide the median fibres more certainly and more freely 
than in the subconjunctival method, without danger of 
cutting the lateral fibres on either side which would leave 
the tendon attached by one corner and might produce 
torsion effects. A conjunctival suture is advisable here, but 
not necessary. It is especially useful if we wish to 
diminish the effect of the operation slightly, and it may be 
taken out in twenty-four to forty-eight hours. 

Shortening and Resection Operations. — The gen- 
eral trend of ophthalmological opinion is in favor of 
resection rather than tenotomy, but there are certain 
logical reasons for preferring one to the other in a given 
case. For instance, if the imbalance is apparently due to 
abnormal weakness of one muscle rather than to the 
abnormal strength of its antagonist, an operation to restore 
the weak rather than to reduce the normal antagonist 
would be indicated. There are two types of operation 
done. The first, by an absorbable suture makes a tuck in 
the tendon. The disadvantages of the method are that the 
effect is not a large one and that the subconjunctival fold 
of muscle is somewhat unsightly for several weeks. The 
advantages are that it is difficult to overcorrect, the effect 
is permanent and there is no danger of torsion, and if the 



252 REFRACTION AND MOTILITY OF THE EYE. 

suture slips no great harm is done. The second method 
excises a portion of tendon and sutures the ends together. 

This has a greater effect and produces no torsion, but 
if the sutures cut through, as occasionally they do, the 
effect is that of a tenotomy and exaggerates the original 
defect. 

The Shortening — A triangular flap of conjunctiva is 
laid back over the insertion of the tendon, an opening in 




Fig. 95. 



the capsule of Tenon made either above or below the muscle 
and two small strabismus hooks passed completely under it. 
These can be drawn apart by an assistant or the twin hook 
of Yalk (Fig. 96) or the tendon tucker of Todd employed 
instead. One suture is then employed to bring the distal 
portion of the tendon up to the insertion, as shown in the 
illustration, the intermediate part forming a loop or fold. 
The suture material may be fine catgut (00) or preferably 
silk which is removed after a week. The conjunctiva may 
be brought together with a single stitch. This operation 
may be done under cocain and the reaction is not very 
great. 



HETEROPHORIA. 253 

Iii the resection operation the tendon is laid bare 
in the same way and a suitable portion being exsected, the 
distal end is prevented from retracting by forceps and 
united to the stump by silk sutures. If silk sutures are 
used for either the resection or the shortening, they are 
best passed through the conjunctiva and muscle. The 
knots are thus readily accessible when the time comes for 
their removal and the same suture also serves to close the 
conjunctival wound sufficiently. 

Esophoria. — Esophoria is characterized by an undue 
convergence of the visual lines for distance or near objects, 
or both, and may be due to the action of one or more of 




Fig. 96. 

these causes, the first four being various stages of the same 
process. 

1. Overstimulation of convergence from accommoda- 
tion strain; 2. Excessive power of interni — convergence 
excess; 3. Defective externi — divergence insufficiency; 4. 
Combinations of 2 and. 3; 5. Monocular imbalance due to 
hypertrophied internus or atrophic externus in one eye 
alone. 

As these conditions vary considerably in their symp- 
tomatology and treatment, they should be studied sep- 
arately. 

First Stack : Accommodative Esophoria. — This is 
evident only when accommodation is active but in time 
leads to anatomical changes causing permanent esophoria. 



254 REFRACTION AND MOTILITY OF THE EYE. 

It will always be more marked at the near point while it 
may possibly be entirely wanting at the far point of the 
eye. 

In pure cases the Maddox rod and vertical prism 
tests all show more or less homonymous diplopia, which is 
diminished or abolished by correction of refraction or 
atropin. 

Fusion tests show a slight increase in convergence 
proportion as compared to divergence. 

Tromometer tests show normal voluntary rotation of 
the eyes. 

Equilibrium tests at the near point, like the screen test 
and the vertical diplopia test, show a very marked esophoria 
which diminishes greatly under proper refractive correc- 
tion. It must not be forgotten that a convex sphere or 
cylinder is ordinarily centred for distant vision, but that 
in near work the visual lines pass to the inside of the optical 
centres. 

Such a lens not only reduces esophoria by lessening 
accommodation, but also has the further effect of a prism 
base out. . 

The treatment consists of a careful correction of the 
refractive error, generally under a cycloplegic, and the use 
of the full correction for all close work. It may be 
judicious in many cases to reduce this correction for dis- 
tance with a view to more satisfactory sight. 

Second Stage: Convergence Excess results from 
the hypertrophy of the interni, which follows the preceding 
accommodative overstimulation. 

Position of rest shows some convergence of visual lines 
as shown by Maddox rod and vertical diplopia tests, the 
homonymous diplopia persisting in spite of refractive cor- 



HETEROPHORIA. 255 

rection, and increasing when the gaze is turned to the 
right or left. Prism convergence is increased and diverg- 
ence somewhat reduced, the proportions instead of 3 to 1, 
being 4 to 1 or 5 to 1, and holding this ratio pretty per- 
sistently. Tests at near points show esophoria, 

Tropometer tests show an increased inward rotation of 
the eye without any diminution of the outward rotation, 
about 55 degrees in and 45 out. 

The treatment consists in first, the full correction of 
the refractive error under a cycloplegic which will, in many 
cases, relieve all subjective symptoms and in time allow 
the muscles to assume their normal size and power. 
Exercise of the externi with prisms is of rather doubtful 
value, but it is at times helpful. Operative treatment 
should be a last resort except in extreme cases, since while 
the immediate results in the way of relief of pain are 
always good, the remote ones of creating an exophoria by 
over-effect should not be forgotten. Logically the prefer- 
able operation would be a slight tenotomy of both interni. 
A shortening of the externi, while more formidable and 
conspicuous for a longer time, is equally satisfactory from 
a clinical standpoint. 

Third Stage: Divergence Insufficiency with 
Convergence Excess is generally the result of a breaking 
down of the externi by overaction of the interni which be- 
come hvpertrophied in the process. In many of the cases 
we can trace the progress beginning with a pure accommo- 
dative esophoria followed by, at first a convergence excess 
and later by the divergence insufficiency. 

The symptoms are very marked at a distance and in- 
crease near at hand. 

These are the cases which pass over into convergent 
squint. 



256 REFRACTION AND MOTILITY OF THE EYE. 

Position of Rest. — Marked homonymous diplopia by 
all tests. 

Fusion Tests. — Divergence very much reduced and 
badly maintained, perhaps 1 A or 2 A • Convergence very 
much increased 30 A to 60 A- After testing the interni, 
spasm often develops so that for the time divergence dis- 
appears altogether and a spontaneous diplopia is present. 

Tropometer tests show a marked increase of inward 
rotation and a marked deficiency in the outward, 55 in and 
35 out, for instance, in each eye. Operative treatment is 
the only one that is likely to be helpful in these cases, 
though, of course, the refractive error as the probable first 
cause should be carefully corrected. The operation will 
consist of shortening or advancement of the underacting 
muscles or tenotomy of the overacting, according to 
whether the convergence excess predominates or the 
divergence insufficiency. Not infrequently both will be 
necessary. If tenotomies are done, they should be guarded 
ones for fear of producing overeffect and as a rule it is 
better to get all possible effect from a shortening of the 
externi and some time later tenotomize the interni very 
carefully as much as seems necessary. 

Divekgence Insupficiency is rarely due to a weak- 
ness of the externi without any change in the interni. 
Naturally its symptoms will be more noticeable in those 
tests of the eye which call for the greatest action of the 
externi, namely distant vision, and will diminish or dis- 
appear at the near point. 

Position of Rest. — Maddox rod, cover test and vertical 
prism, all show marked homonymous diplopia, increasing 
eyes right or left, which diminishes or disappears when 
the object is brought up to the reading distance. 

Fusion tests show a diminished divergence, the externi 



HETEROPHORIA. 257 

being able to overcome only very weak prisms and this 
diminishes rapidly under test. The interni are ordinarily 
about the average normal, certainly there is no great in- 
crease in convergence and not infrequently it is diminished, 
but the proportion of convergence to divergence increases 
from 3 or 4 to 5 or 6. Very often the externi becomes so 
fatigued during the fusion tests that afterward there is a 
homonymous diplopia for a time with a red glass or even 
the naked eye. 

Tropometer tests show a lessened outward rotation of 
each eye with no increase or only a slight increase in the 
inward movements. 

Treatment consists of refractive correction as a matter 
of routine without any expectation that it alone will be 
enough. Prism exercises are hardly worth a trial and the 
wearing of prisms, base out, with the idea of helping the 
externi, will be only temporarily beneficial, since the 
ultimate effect will be to exercise the interni and so 
exaggerate the original condition. Operative treatment 
may be anticipated from the first and the logical operation 
is a shortening or an advancement of the externi since a 
tenotomy on the interni which are already not overstrong 
might improve conditions for distance, but would sub- 
stitute new difficulties at the near point. 

OVERACTION OF ONE ADDUCTOR OR UNDERACTION OF 

one Abductor. — These conditions are sometimes con- 
genital defects of muscular insertion or bulk and sometimes 
the result of injury or disease and very often of previous 
operation on the eye muscle. Evidently, if only one 
muscle be involved, the esophoria will be more marked in 
the field of this muscle. For instance, if the right internus 
be involved, there will be by the rest tests homonymous 
diplopia which will increase when the strong muscle is in 

17 



258 REFRACTION AND MOTILITY OF THE EYE. 

action as in turning the eye to the left, and disappear 
entirely when the strong muscle is relaxed as in turning 
the eye to the right. But the same thing will occur if, 
instead of an overacting right internus, we have an under- 
acting right externus. 

The fusion tests are of no help in finding which muscle 
is at fault, for the fusion power, whether in divergence or 
convergence, is limited by the capacity of the weaker muscle 
of the pair. The right internus cannot manifest its full 
power, because long before that is reached the left internus 
has given out and the left eye diverged. 

The tromometer is the most reliable test in these cases, 
because it indicates definitely whether both muscles or only 
one are involved, and exactly which. For example, E.E. 
50 x 45, L.E. 60 x 45, shows an overacting left internus; 
E.E. 50 x 45, L.E. 50 x 40, an underacting left externus and 
R.E. 50 x 45, L.E. 60 x 35, would indicate both together. 

The treatment of these cases, if they occasion symp- 
toms, is operation on one eye, either weakening the over- 
acting strong muscle or strengthening the underacting, or 
both. 

Exophoria denotes a tendency of the visual lines to 
diverge for objects at a distance or near at hand, or both. 
Except for the intervention of the fusion compensation, it 
would cause diplopia and even when overcome, it causes 
symptoms, depending on the amount of defect and the 
proportion of compensating power. If the exophoria is 
due to the presence of interni which are long and lax, but 
not wanting in strength, the diplopia will be overcome by 
a very slight effort and may cause no symptoms of any kind. 
On the other hand, it may be due to the actual muscular 
imbalance, either over-powerful externi or weak interni, or 
both together. In either case the patient may be able to 






HETEROPHORIA. 259 

fuse distant images without obvious effort, but in near 
vision his eyes have to be converged through a wider arc 
than normal by interni which are either intrinsically weak 
or burdened with too much opposition by strong externi. 
Xear vision is a matter of abnormal effort and if at all 
continuous, results in diplopia, with the suppression of one 
image or in reflex symptoms in direct proportion to the 
effort involved. The first is painless, but deprives the 
individual of the advantages of stereoscopic vision. He 
sees everything in the flat and has but slight facility in 
estimating and measuring. If, however, fusion is con- 
stantly attempted, the patient suffers all the disabilities 
already referred to. In exophoria then, the subjective 
symptoms are slight in distant vision and increase progres- 
sively with the closeness and continuance of near work. 

As the treatment depends entirely on the cause, a very 
careful application of the tests referred to in the previous 
chapter must be made. 

Accommodative Exophoria. — Just as esophoria may 
be due to excessive accommodation, so exophoria may be due 
to lack of accommodation. The myope, for instance, who 
sees best with his accommodation completely relaxed, not 
infrequently shows an exophoria for distance which is very 
much more marked in near work which calls for increased 
convergence with little, if any, accommodation. 

Position of Rest. — All tests show cross diplopia, not 
necessarily very great for distance, but increasing markedly 
nearer at hand and disappearing entirely when proper 
refraction is secured. 

Fusion tpsts are not very reliable in myopia because 
fusion depends to a great extent on sharp vision which can 
only be secured in many cases by strong concave glasses. If 
tested without glasses, divergence is apparently somewhat 



260 REFRACTION AND MOTILITY OF THE EYE. 

above normal. A portion of the prism overcome, simply 
allows the eye to assume the position of rest without 
diplopia and does not necessitate any muscular effort, and 
if we add to this the further prism which is overcome by 
actual muscular contraction even if it be subnormal, the 
apparent prism divergence is increased. In measuring 
prism convergence the eyes start from a position where 
some of the convergence energy is already expended to 
secure single vision and in addition is deprived of the 
additional stimulus which should come from sharp vision 
and some accommodation. Consequently, convergence is 
generally subnormal. 

It might seem at first sight as though fusion ought to 
be tested through proper myopic lenses. If, however, the 
myopia be a high one, the instant the eye diverges from the 
optical centre of the lens, the lens acts as a prism, base out, 
and nullifies part of the prism used in the test. In con- 
verging, the eye looks to the inner side of the optical centre 
of the correcting lens which here acts like a prism, base in, 
and nullifies the action of the prism used in the test. The 
prismatic action of the myopic glass increases progressively 
the further from the centre, and consequently, as the eye 
normally converges far more than it diverges, the error in 
the estimate of prism convergence is far greater than in 
prism divergence. 

The tromometer test will show a normal rotation in all 
directions in exophoria due to lack of accommodation. 

Treatment. — Correct refraction in full under cyclo- 
plegic. 

Convergence! Insufficiency is due to lack of power 
or innervation of the interni and the symptoms will 
naturally be slight or wanting at a distance and increase 
progressively as close work throws strain on these muscles. 



HETEROPHORIA. 261 

Equilibrium tesfs negative for distance or showing very 
moderate X diplopia. Very marked X diplopia for 
near, cover test showing distinct redress movement inward. 

Fusio?i Tests. — Divergence normal or increased; con- 
vergence much reduced and badly maintained. X diplopia 
very frequently after fusion tests showing temporary 
exhaustion of interni. 

Tropometer shows outward rotation normal or slightly 
increased. Inward rotation reduced and growing less on 
repeated trials: 40° inward by 45° outward is a fair 
example. 

Treatment. — Correction of refraction especially if 
myopic. These are the only ones that are markedly 
benefited by exercise several times a week with prisms base 
out and carried to the point of fatigue, conjoined with rest 
from near work and attention to the general health. If 
improvement does not result or is not reasonably permanent, 
a shortening of one or both interni is the logical resort. 
Slight tenotomy of the externi, while not logical, yields 
good clinical results, but is very likely to cause a divergence 
insufficiency. 

Divergence Excess is due to overaction of the externi, 
the interni, being normal. The s} r mptoms are therefore 
likely to be more noticeable at distance than in near vision. 
Subjective symptoms often nil. 

Equilibrium Tests. — Very marked X diplopia for 
distance by all tests. Much less marked or wanting 
entirely for near points. 

Fusion Tests. — Prism divergence much increased, 10 A 
or over. Prism convergence good, but not proportionate. 

Tropometer tests show increased outward rotation with 
normal inward rotation ; 50 x 50 is a fair example. 

Treatment. — Complete correction of refraction, espe- 



262 REFRACTION AND MOTILITY OF THE EYE. 

daily if myopic. Exercises calculated to weaken the 
externi and develop the intern! may be tried. Operative 
treatment is seldom necessary, the logical choice being 
slight tenotomy of both externi. Clinically there is no 
objection to a moderate shortening of one or both interni. 

Divergence Excess with Convergence Insuffi- 
ciency. — This implies overacting externi and weak interni. 
The symptoms will be manifest not only at distance, but 
will be increased in near vision. These are the cases that 
easily pass over into divergent squint. 

Equilibrium Tests. — Marked X diplopia by all tests 
at distance and increased in close work. 

Fusion tests show increased prism divergence and 
prism convergence much reduced or wanting entirely. 
Where present, it is badly maintained and frequently after 
testing the fusion powers the interni are so exhausted that 
spontaneous X diplopia is present. 

Tromometer tests regularly show increased outward 
rotation and reduced inward; 35° in x 55° out in each eye 
for example. 

Treatment consists of tenotomy of overacting externi. 
Systematic exercise of interni and shortening, if necessary, 
later on. 

Hypertrophy of one Externus or Weakness of 
one Internus may be due to a congenital anomaly of 
strength or insertion, disease, or is often the result of 
operation for squint in childhood. The symptoms will be 
most marked in the field of the abnormal muscle, and 
either slight or altogether wanting in motions of the eyes 
which do not involve the abnormal muscles. Eor instance, 
in overaction of right externus. 

Equilibrium test shows X diplopia for distance and 
near, which increases on turning the eyes to the right and 



HETEROPHORIA. 263 

diminishes or disappears on turning eyes to left, which 
might mean an overacting right externus or an underacting 
left internus. Xear tests show diplopia, and if fusion is 
possible it is much easier on carrying the object to the left 
so as to use the normal right internus and left externus. 
Fusion tests are of no value in monocular imbalance for 
reasons already explained. 

Tropomcter tests are of the utmost value, showing at 
once the muscle involved. For instance, a reading of E. 
50° in x 45° out, L. 50° in x 55° out, shows at once an 
overaction of the left externus, while E. 50° in x 45° out, 
L. 40° in x 45° out, would plainly indicate a defective left 
internus. 

The treatment of such a condition is essentially 
operative, since there is no way of exercising a single 
muscle without a corresponding effect on its fellow in the 
other eye ; and the operation selected should be a shortening 
of the defective muscle or a tenotomy of the overacting one 
in proportion to the correction desired. 

Hyperphoria. — We should group under this head all 
the vertical deviations of the visual axes since the diplopia 
is the same whether one eye be turned up or its fellow 
slightly down, one image appearing directly over the other. 
This is a very important muscular defect since the facility 
in fusing vertical images is very slight indeed, the prism 
power of the elevators being only 2° and that of the 
depressors only 3° or 4°. A comparatively slight vertical 
error then completely abolishes the fusion power of the 
eyefl and allows the eyes to assume the position of rest. In 
this way a slight hyperphoria will prevent the suppression 
of an esophoria or exophoria which would not be noticed at 
all if the images were on the same level. In such cases the 
lateral diplopia is often so much greater than the vertical 



264 REFRACTION AND MOTILITY OF THE EYE. 

that the latter is unnoticed when it is by far the most 
needful of attention. It may be due to overaction of an 
elevator or underaction of a depressor in one eye, or the 
reverse condition in the other eye, and it will often tax our 
ingenuity to ascertain which muscle is involved. For 
purposes of record it is the custom to designate a vertical 
diplopia as a right diplopia or a left, according as the image 
in the right or left eye is the lowest. 

Equilibrium tests (prism base in, Maddox rod, red 
glass) show a vertical diplopia. If the right eye image is 
the highest (left diplopia), it indicates that the right eye 
is pointing lower down than the left. If the vertical 
diplopia increases on looking upward we have to do with an 
underacting right elevator or overacting right depressor, 
But the defect may be in the other eye, as for instance, an 
overacting left elevator or underacting left depressor, 
causing the left eye to point too high. The effect that the 
obliques have on the elevation and depression of the eyes 
must not be forgotten. However, if we can exclude any 
involvement of the oblique muscles by the crossed Maddox 
rods or the Maddox double prism, we have simplified our 
problem materially. A vertical diplopia which increases 
greatly when the eyes are turned in one direction, and 
diminishes or disappears in the opposite, comes under the 
head of paralysis rather than hyperphoria (see page 303). 

Prism fusion tests in hyperphoria are not helpful, 
since even in normal eyes fusion of vertical images is slight 
and variations from the standard are usually slight except 
in conditions bordering on paralysis. Moreover, since each 
eye has four muscles which may be concerned in hyper- 
phoria, a test which is not capable of distinguishing their 
separate actions is more confusing than helpful. The 
increase of the diplopia as shown by the equilibrium test 



HETEROPHORIA. 265 

when the eyes are moved in various directions is somewhat 
more helpful. For instance, a left diplopia increasing as 
the eyes turn upward and diminishing as they turn down- 
ward would evidently be due to an overaction of the right 
elevators or underaction of the left depressors. 

The tropo meter is the most reliable and exact method 
of identifying the muscle at fault, but even with this much 
more than the usual care must be taken to obtain a position 
of the head with the eyes on a level and to be certain that 
the head is not moved during the tests. As we have seen, 
the upward and downward rotation of the eyes varies 
greatly with the position of the head whether tilted forward 
or backward, but it ought to be equal in both eyes. If we 
find that the right eye rotates upward 30° and down 35°, 
while the left rotates upward 20° and down 35°, there is 
evidently a deficiency of the left elevators. If the rotation 
is 30° x 45° it reveals a deficiency of left elevators. 20° 
x 55° shows overaction of left depressors, 40° x 25° shows 
overaction of right superior and underaction of right 
inferior, 20° x 45° shows underaction of left superior and 
overaction of left inferior. 

Furthermore, by aid of the tropometer we can 
definitely measure the rotating ability of any one of the 
straight muscles by placing the eye in such a position as to 
nullify the possible elevating and depressing action of the 
obliques. 

For instance, when the head is in a symmetrical 
position the right eye is elevated by the superior rectus and 
inferior oblique, but if we turn the head in the headrest so 
that the eye is turned slightly outward, the obliques produce 
only a torsion effect and any elevation of the eye must be 
due to the superior rectus unaided, while any depression 
must be due to the inferior rectus. 



266 REFRACTION AND MOTILITY OF THE EYE. 

Treai?nent. — Hyperphoria is a very important element 
in heterophoria and very often overlooked, and since the 
fusion power over vertical diplopia is very slight indeed, 
the equilibrium tests have in this variety of imbalances an 
importance that they attain in no other. As we have seen, 
when the error is slight, the patient can compensate by 
tipping the head to one side so as to elevate one eye and 
depress the other, but this throws a constant strain on the 
obliques of keeping the vertical planes of the eyes from 
being also tilted. 

The fusion powers are so slight and so incapable of 
development by exercise in this condition that exercises are 
of no special value. This fact is of particular importance, 
however, in the application and the prescription of prisms 
for permanent use, since prisms which place the images in 
the same plane are seldom very strong and the effect is 
permanent. For this reason prisms base up or down in 
either eye, as may be required, are more useful in this 
condition than in any other. 

Operative treatment is seldom called for. Since the 
straight muscles are the ones chiefly concerned in elevating 
and depressing the eyeballs. and since they are the most 
accessible, operations are confined to them. By aid of 
the tropometer we can ascertain which muscle is at fault 
and whether by overaction or underaction, so that theoret- 
ically a tenotomy of the overacting or a shortening of the 
underacting muscle would be indicated. Clinically, how- 
ever, so long as the eyes are in the same plane, it makes no 
practical difference in most cases whether they go up as 
many degrees as they should, while it is of manifest impor- 
tance that they should be capable of a considerable depres- 
sion. Ordinarily, if a tenotomy is done, it should be on the 
superior rectus of the hyperphoric eye, since this will not 



HETER0PH0R1A. 267 

limit the depression of the eyes as a pair, and since one is 
not so apt to get overeffeet as from a tenotomy of the 
stronger inferior. For similar reasons a shortening, if 
done, should be done on an inferior of the hyperphoric eye, 
since this increases the ability to look downward and since 
the inferior is more easily reached in its continuity than 
the superior. 

Dextrophoria and laevophoria have been suggested by 
Valk as terms to describe a condition in which both eyes 
are capable of abnormal rotation toward the right or 
left as the case may be, the movement in the opposite 
direction being restricted. In dextrophoria for instance, 
which is by far the most common, the patient can rotate 
his eyes toward the right 60° and to the left perhaps, only 
40°. His position of rest is, then, with his visual lines 
parallel, but to the right and in looking at objects directly 
in front he is much more comfortable with his head rotated 
slightly to the left. It is difficult to account for, except on 
the theory that definite movement of the eyes is, in most 
occupations, rather to the right than the left. The position 
of the paper in writing at a desk tends toward a dextro- 
phoria ; in reading, we move our eyes steadily from left to 
right and then begin a new line by a single brief movement 
to the left; the things that a man, be he laborer or student, 
uses oftenest, he keeps within reach of his right hand and 
in referring to them he is constantly turning his eyes to 
the right. 

The visual lines being parallel and the fusion powers 
generally normal, the condition is commonly overlooked, 
unless so extreme as to cause a marked turning of the head, 
but a routine use of the tropometer shows it to be very 
common, a typical record being E. 40° in x 60° out, L. 60° 
in x 40° out. Since dextrophoria is so easily compensated 



268 REFRACTION AND MOTILITY OF THE EYE. 

for by turning the head, it is so absolutely without subjec- 
tive symptoms, even if it be not an actual advantage to a 
right-handed man, that it had better be left entirely alone. 

It very often, however, results from other imbalances 
when it must be treated more carefully. For instance, 
suppose a patient whose right internus has been cut for 
squint, is paralyzed or congenitally defective. When he 
looks to the left he has a cross diplopia which vanishes to 
the right and* he soon gets the habit of carrying his head 
in this easy position. Ordinarily, this will cause no dis- 
comfort, but if the left internus is so weak that it cannot 
follow the right externus to its position of greatest ease 
the visual lines are evidently divergent and the case must 
be treated as an exophoria. If, on the other hand, the left 
internus overbalances the right externus, the condition is 
an esophoria and must be treated as such. 

Similar reasoning applies to the conditions known as 
anaphoria and kataphoria in which the visual lines are 
parallel to each other but directed up or down with regard 
to the horizontal plane of the body. 

In the first, owing to congenital abnormalities, usually, 
the eyes tend upward and the individual must go about with 
his chin on his chest, that his eyes may look in front and 
yet remain in the position of rest. In the second, the chin 
is held in the air and the body arched backward, but unless 
extreme, neither causes more than cosmetic difficulty and 
both should be let alone, owing to the extreme difficulty of 
securing the same operative effect on both eyes. Suitable 
prisms are much more likely to be beneficial. 

Cyclophoria is the condition in which the oblique 
muscles are no longer able to keep the vertical meridians of 
the retinas parallel. For instance, if the superior oblique 
of the left eye is defective, the vertical meridian rotates 



HETEROPHORIA. 269 

outward above and inward below. If the object of regard 
be a vertical line the right eye sees it vertical, while in the 
left the top appears tilted to his left and the bottom to his 
right. 

The composite picture is that of an oblique cross and 
when the object of regard is of a nature not so simple as a 
single line, the distortion and confusion are very marked. 

The diagnosis can be made by the method described in 
the previous chapter with the double Maddox rod, the Mad- 
dox prism or the clinoscope. 

The treatment consists in exercise of the torsion 
powers with the same instruments or if this proves 
ineffectual, operative measures are strongly advocated by 
Stevens. Inasmuch as the oblique muscles themselves are 
inaccessible the internus is made to assume a torsion 
function in this manner. We have seen that the internus 
has a broad fan-like insertion, one-half above and one-half 
below the mid-line of the cornea. Now, if the lower half 
of the insertion is cut loose and allowed to slip back, the 
remaining half will evidently pull more on the upper end of 
the vertical plane and so assume in part the function of the 
superior oblique. The same purpose is subserved by 
advancing the upper half without touching the lower and 
the maximum effect obtained by combining the two. By 
reversing the procedure, the internus can be made to replace 
in part the inferior oblique. The treatment of cyclophoria 
is at present in the field of experimental surgery and should 
not be attempted except in case of absolute necessity and 
after the most careful testing. 



CHAPTER XIII. 
HETEROTROPIA— STRABISMUS— SQUINT. 

Heterotopia, strabismus and squint are synonymus 
terms and mean that when the patient fixes an object with 
one eye, the other is so turned that the image ceases to fall 
on the macula. Squint is of two varieties: that in which 
the deviation is due to paralysis of one or more muscles, 
which will be discussed in another chapter, and the non- 
paralytic variety. In paralytic squint the deviation is per- 
ceptible only when the gaze is turned in such a direction as 
to bring the paralyzed muscle into action, and increases as 
the eyes are turned in this direction. In non-paralytic 
squint, on the other hand, the inclination of the visual lines 
toward each other is the same in every direction of the gaze. 
If we have a squinting patient regard a candle, he fixes it 
with one eye and turns the other in toward his nose. This 
deviation of the squinting eye is known as the primary 
deviation. If, now we cover this eye, the patient im- 
mediately fixes the candle with the other, and if we look 
behind the screen we shall find that this eye now deviates 
in exactly as much as its fellow did formerly. This 
deviation is known as the secondary deviation. The fact 
that the inclination between the optic axes is a constant one, 
the primary and secondary deviations being equal, has 
caused the term concomitant to be applied. In paralytic 
squint the primary and secondary deviations are not equal, 
as we shall see. 

Measurement of Squint. — We have the patient look at 
a distant object and place a dot on the lower lid opposite 
(270) 



HETEROTROPIA. 



271 



the margin of each pupil. We then compel him to fix the 
object with the squinting eye and again, mark the position 
of the margin of the pupils by dots. This distance between 




Fig. 97. 




the two marks on the squinting eye measures the primary 
deviation and that between the marks on the other, the 
secondary. In concomitant squint they are always equal. 
On similar principles is the strabismometer (Fig. 98), 



272 REFRACTION AND MOTILITY OF THE EYE. 

an ivory instrument, fitted to the curve of the lower lid. 
The patient is compelled to fix an object with the squinting 
eye and the centre of the instrument placed below the 
centre of the pupil. When he fixes the same object with the 



A 




Fig. 99. 



other eye, the amount of deviation in or out can be read 
off upon the scale. Both these methods are very crude and 
inexact. 

If we place the squinting eye opposite the centre of a 
perimeter and have the patient fix the candle with both 
eyes uncovered, the squinting eye will deviate one way or 






HETEROTROPIA. 273 

the other from the centre. If we carry a candle along the 
arc of the perimeter, keeping one eye exactly behind it our- 
selves till the reflection appears exactly in the centre of the 
squinting eye, we shall have found the fixation point; and 
the number of degrees by which it varies from its normal 
position F, will be the measure of the squint in degrees. 

These methods are all relative — squint cannot be 
measured exactly since it is often a variable quantity. For 
instance, if we have a patient fix a distant object, we find 
a very slight deviation or perhaps none at all, but if we 
measure him at the perimeter where he uses his accommoda- 
tion, it is much more noticeable. In practically all cases of 
squint, changes, either primary or secondary, will be found 
in the muscles of the eye affecting their rotatory power, and 
these can be accurately measured by the perimeter or better 
still, by the tropometer as in latent squint. This is 
especially important when operative interference is con- 
templated, for an amblyopic eye is apparently always the 
squinting one, since fixation is always performed with its 
fellow. As a matter of fact, however, the muscular changes 
are often greatest in the eye which does not squint. 

We should expect that when one eye fixes an object, its 
image will be formed at the macula and since its fellow 
deviates from the correct position its image must fall else- 
where and therefore, cause a diplopia. This does occur in 
paralysis and in the very early stages of concomitant squint, 
but the patient, by a psychical effort soon learns to keep his 
attention on the distinct macular image and to disregard 
the less distinct non-macular one. After a time the habit 
gets so fixed that even when both eyes see well it is very 
difficult or impossible to make the patient conscious of his 
diplopia. A squinting patient evidently cannot have 



274 REFRACTION AND MOTILITY OF THE EYE. 

stereoscopic vision and consequently his ideas of form, size 
and distance are much less accurate than normal. 

The visual acuity in strabismic patients varies greatly. 
In many cases the acuity of the two eyes was originally 
equal, but after the patient gets the habit of suppressing 
the image in the squinting eye, it fails greatly from simple 
lack of use. This failure is called amblyopia exanopsia. 
In such an eye the acuity is capable of very great improve- 
ment, sometimes up to normal, by systematic exercise. In 
other patients there may be a congenital amblyopia in one 
eye which compels the patient to fix with the other, or the 
refractive error may be so great in one that any distinct 
vision is impossible without glasses. This causes a prac- 
tical amblyopia exanopsia from birth. If corrected very 
early during the plastic period of childhood, the amblyopia 
may grow greatly less, but, if neglected, the amblyopia 
becomes permanent. 

The amount of amblyopia varies greatly; in some 
patients vision is reduced to absolute inability to count 
fingers or less, and the eye has lost all its power of fixation, 
so that if its fellow is covered it maintains its position. 
In others a vision of 20/200 up to 20/30 is found and 
occasionally a patient is seen where the visual acuity qf 
both eyes is practically equal. As a rule, however, the 
vision of the squinting eye fails steadily with the duration 
of the squint, from lack of use. It is, however, a macular 
amblyopia, since while incapable of distinct central vision, 
the field of vision is generally normal, the brain taking 
cognizance of peripheral impressions. 

Apparent Squint. — There are a number of patients 
whose eyes diverge or converge to all appearances, but in 
whom, if one eye be covered, the other maintains its posi- 
tion showing that both images were formed at the macula 



HETEROTROPIA. 275 

and that the vision was binocular in spite of the apparent 
squint. 

These cases are to be explained in this way. In the 
ideal eye the optic axis which is perpendicular to the 
cornea and the lens should pass through the macula and 
coincide with the visual axis. In most eyes, however, the 
macula is a trifle to the outer side of its proper location, so 
that a line from it to the object of regard passes through 
the nodal point and cuts the cornea a little to the inner 
side of its centre. But we judge the position of the eye 
from the cornea; therefore, if the macula is too far to the 
temporal side the visual lines will be nearer to the inner 
side of the cornea and the eyes appear to diverge, while if 
the macula were to the nasal side of the optic axis, the 
visual lines would pass to the outer side of the corneal 
centre and the eyes appear to converge. The amount of 
this apparent squint is determined by the size of the angle 
between the optic axis and visual lines known as angle 
gamma. 

That the squint is apparent and not real is readily 
determined, since there is no movement of redress when the 
apparently fixing eye is covered, provided always that 
both have good vision. The size of the angle can readily 
be measured on the perimeter, the patient fixing a candle 
flame in the centre of the perimetric arc with one eye thus 
locating its visual line. The surgeon moves his eye along 
the arc with a small light till the reflection appears exactly 
in the centre of the cornea, when the optic axis perpen- 
dicular to the corneal centre must be exactly here and the 
degrees can be read off on the arc. 

There are many different terms applied to concomitant 
squint most of which define themselves, as for instance 
convergent, divergent and vertical. In alternating squint 



276 REFRACTION AND MOTILITY OF THE EYE. 

the vision is about equally good in both eyes and sometimes 
the patient fixes with one and squints the other and again 
reverses the process. A periodic squint is one which is 
perceptible at times and again disappears. This generally 
occurs in the early period, the squint later becoming a 
constant one. 

The etiology of squint is a vexed question. We have 
seen in a previous chapter that the ideal eyes are so muscled 
that in a position of rest their visual lines are parallel and 
that where this ideal balance is not present, the equilibrium 
is maintained by the fusion centres of the brain. These 
are the two factors governing the presence or absence of 
squint. If the muscle balance is normal, binocular vision, 
takes place without the stimulation of the fusion centres. 
If the muscle balance is abnormal while the fusion impulse 
is powerful, we have a latent squint which may or may not 
cause symptoms. If the fusion compensation is wanting or 
not great enough for the imbalance, we have an actual 
squint, the variety of which depends on the variety of 
muscle imbalance, and is periodic or constant according to 
the amount of fusion impulse available. 

Let us consider these basal factors and some of the 
conditions which may interfere with the normal working of 
one or both. 

Congenital Muscular Anomalies. — We have absolutely 
no means of knowing the actual muscular balance at birth 
and during early childhood, but bilateral bodily structures 
are seldom or never absolutely symmetrical, and there is 
every reason to suppose that most children have some 
muscular imbalance. But there are certainly in some 
cases congenital gross defects in the origin and insertion of 
one or more muscles, or abnormalities in the bony orbit due 
to asymmetrical bony development or disease, or injury at 






HETEROTROPIA. 277 

birth, which would preclude parallelism of the optic axes 
when the eyes are at rest. On this theory only can we 
account for those not infrequent cases of squint in which 
the muscular defect is in the fixing instead of the squinting 
eye. We should expect nearly all infants to squint more or 
less and most of them do from time to time to the con- 
sternation of parents. But unless the squint is great 
enough to be a cosmetic defect, it is not observed, for until 
they begin to fix objects we have no means of testing the 
position of the eyes except by the position of the corneal 
images which is a very inaccurate one, and any infant might 
have a squint of 5° to 10° without detection. The eyes 
converge and diverge at various times till the brain having 
developed sufficiently to be conscious of double images, a 
fusion impulse compels the eyes to coordinate. Where the 
muscular error is small and the fusion impulse strong, the 
squint becomes a latent one. On the other hand, where the 
anatomical defect is a large one and the fusion impulse 
weak, the squint becomes a fixed one. It is to be remem- 
bered that the stimulus to fusion is greater in proportion 
to the proximity of the images to the macula ; consequently, 
in very great deviations where one image falls in the 
periphery of the retina, the fusion impulse is weak or may 
possibly not develop at all. If the fusion is possible, but 
requires too much effort, the brain finds it much easier to 
suppress one image. Then it is anatomically much easier 
to fuse images in one direction than in another. For 
instance, vertical diplopia can be compensated for very 
poorly. The divergence power of the eyes is not much 
greater so that nature has great difficulty in rectifying a 
convergence, while a divergence of the optic axes is rectified 
with ease. For these reasons convergent squint is very 
much more common in children than any other. The 



278 REFRACTION AND MOTILITY OF THE EYE. 

relative frequency of vertical squint in early childhood we 
know nothing about, because we have no means of detecting 
low degrees of squint applicable to children. For these 
reasons we must think that most of the squints that develop 
within the first two or three years of life are probably due 
to congenital anatomical defects which are beyond the 
fusion power's correction or which even prevent the develop- 
ment of that power. But that comparatively few cases of 
squint are due either to congenital imbalance or failure to 
develop fusion power, is indicated by its relative infre- 
quency in blind asylums among patients who have been 
blind since very early infancy and can never have developed 
any fusion power. 

There is another type of cases which is not evident till 
later in life at the age of five or six, long after the fusion 
power is ordinarily established. If it were due to a con- 
genital abnormality of muscle and failure to develop the 
fusion faculty, it would have appeared earlier, and it is 
evidently due to a muscular anomaly not congenital, but 
acquired, and a suppression of the fusion faculty which was 
previously developed. 

Since muscular changes are the primary conditions, let 
us consider the conditions which may cause changes in 
muscular balance. 

Refraction. — Donders long ago pointed out the inti- 
mate association between refractive conditions and squint. 
As we have previously seen, there is a very intimate relation 
between convergence and accommodation, and the individ- 
ual who through hyperopia or astigmatism has to over- 
accommodate, automatically overconverges. Except for the 
sufficiency of the fusion compensation, all such would 
finally pass from the condition of latent to that of fixed 
squint. Even in middle life exercise of the interni causes 



HETEROTROPIA. 279 

hypertrophy to a much greater extent than does exercise of 
the externi and in the plastic period of childhood the 
hypertrophy in muscle is not temporary but often a perma- 
nent increase in bulk and strength. Where this hyper- 
trophy overbalances the fusion ability, double vision occurs 
for brief periods long before any squint is perceptible, 
especially at periods of unusual strain as in school. Each 
failure to fuse weakens the fusion impulse, and the con- 
vergence meeting less resistance increases rapidly, the 
patient learning to suppress one image completely, and the 
fixed squint has been established. But this does not 
explain divergence which occasionally occurs in hyperopia. 
We must bear in mind the effect on a muscle of stimulation, 
which within physiological limits causes hypertrophy, but 
when carried beyond the power of the muscle to respond 
causes it to lose its responsiveness and results in atrophy. 
In childhood when the muscular metabolism is most active, 
hypertrophy generally occurs with convergent squint, but 
in maturer years divergence in connection with hyperopia 
is much more common, especially when the fusion faculty is 
suddenly interfered with by some accident or. injury. In 
childhood, when hyperopia is so high as to be entirely 
beyond ciliary compensation, we have seen that a child 
frequently holds objects very close to the eye to secure a 
large, though indistinct, retinal image, perhaps not accom- 
modating at all. In such a case the strain of converging 
constantly at a point only two or three inches away would 
be tremendous and would certainly lead to hypertrophic 
interni ; if any attempt were made at binocular vision the 
result could hardly be other than that of hypertrophy with 
convergent squint or of atrophy with divergence. But in 
most of these cases the vision is so poor that accommodation 



280 REFRACTION AND MOTILITY OF THE EYE. 

or fusion stimulus is practically nil and often no changes in 
the muscles occur at all. 

The conditions in myopia are very different. Allowing 
that a child is born with normal muscles, his distant vision 
is so much reduced by his myopia that fusion instincts are 
not called into play till the period of close work arrives. In 
a myope of 4 D. for instance distinct vision is only possible 
at ten inches and this with a complete relaxation of accom- 
modation. Here again the normal relation between 
accommodation and convergence is disturbed and the 
impulse to converge is weak, and if the fusion impulse is 
not a strong one divergence occurs. But young people 
notoriously do not hold their work at their far point but 
bring it closer, because they can thus, by accommodating, 
both obtain sharper vision and aid in the convergence 
impulse. The customary fixation point is abnormally close 
and though no squint is present, the inclination of the 
visual lines toward each other is often far greater than in 
those of a squinting hyperope. This results in young 
children in an overdevelopment of the interni. If the 
myopia is a high one, the interni break down under the 
strain of binocular vision so near by and divergence begins. 
Even in the more moderate and stationary myopias, with 
age and increased hours of labor, the interni become more 
and more insufficient, and divergence becomes more fre- 
quent, while in the progressive cases the already overtaxed 
interni are loaded with the increasing proximity of the far 
point and the failure of a sharp fusion impulse through 
fundus disease. 

But very considerable muscular imbalances may be 
present without actual squint provided the fusion power be 
good. Let us consider some of the factors on which fusion 
depends, a sharp image and a normal cerebral reflex action. 



HETEROTROPIA. 281 

Any cause which reduces the visual power of one or both 
eyes diminishes the fusion stimulus and lessens the power 
to compensate for slight muscular imbalance. For this 
reason opacities in the cornea, lens and vitreous and 
changes in the fundus are often followed by manifest squint 
of some sort which was before latent. 

Refractive difficulties when not compensated also 
reduce vision and lessen fusion. 

In hyperopia and hyperopic astigmatism vision is 
better at a distance, the fusion stimulus greater and the 
squint not marked. Xear at hand however, not only is the 
tendency to squint increased by the accommodation strain, 
but the fusion power is much reduced if the near vision is 
defective. In myopia, on the other hand, distant vision is 
bad, the fusion stimulus small and the squint more marked, 
while in near vision the images are distinct, the fusion good 
and the squint often imperceptible. 

The cerebral element in fusion is also very important. 
Some individuals never show any tendency for binocular 
vision and are not annoyed by diplopia even though vision 
be normal in both eyes. 

The condition of amblyopia, so very commonly present 
in squinting eyes, is an interesting one, meaning a reduc- 
tion in vision without any demonstrable lesion. By some 
this is considered congenital and, through the reduction in 
fusion power, the cause of squint, while others consider that 
it is generally secondary to squint and results from lack of 
use and suppression of the image in the squinting eye. 
Retinal hemorrhages are extremely common in the new- 
born and a small macular lesion would easily account for 
a congenital amblyopia. The very high amblyopias in 
which vision cannot be brought above 20/200 are probably, 
but not certainly, congenital. They rarely improve though 



282 REFRACTION AND MOTILITY OF THE EYE. 

a few cases are recorded. The partial amblyopias of 
20/70, 20/50, etc., are very often improved by forced use 
of the amblyopic eye and are therefore amblyopias exan- 
opsia or from disuse. 

There is a very general lay belief that squints develop 
very suddenly early in life as the result of fright, bright 
light, illness of various sorts and other circumstances which 
occur in the life of every child. There is so far as we 
know no scientific basis for such a belief, but most impres- 
sions which are so universal have at least a basis of truth 
and are not to be neglected entirely. Perhaps the fusion 
impulses are thus inhibited for the first time. 

Spontaneous cure of strabismus of the convergent type 
not infrequently takes place after the patient reaches the 
age of twenty-five or thirty. These cures are to be explained 
only on the theory that the squint is not a primary one, 
but was due to overstimulation of convergence in hyperopia. 
As the patient gets older, the elasticity of his lens gets less, 
and he gradually ceases accommodative efforts which no 
longer produce any result. With the lessened accommoda- 
tion the convergence stimulus is also less, the muscles 
gradually lose their hypertrophy, and the muscular balance 
is so nearly restored that no perceptible squint remains. 
Careful testing would probably show that in most of these 
cases a latent squint remains. 

Treatment of Squint. — For purposes of treatment cases 
of strabismus may be divided into two great classes : those 
in which vision in both eyes is relatively good or can be 
made so by glasses, and those in which one eye is so am- 
blyopic as to be practically blind with no reasonable hope 
of improvement. In adults and older children this is not 
difficult to ascertain, but in young children it will very 
often tax one's ingenuity. 



HETER0TR0P1A. 283 

One can obtain an approximation of the visual acuity 
in each eye by placing on the floor small white marbles and 
noting the distances from which the child is able to go to 
them directly with each eye. If the vision in both eyes is 
fairly good, the prognosis for cure without operation is 
much better than when one eye is highly amblyopic. But 
we must not decide that an eye is hopelessly amblyopic in a 
young child without repeated examinations. Both eyes 
should now be thoroughly atropinized and the refraction 
exactly estimated which can easily be done by aid of the 
retinoscope. In many cases the squint will entirely dis- 
appear while the patient is under the mydriatic which is 
proof that it is essentially an accommodative squint. In 
this case the constant wearing of the full correction with 
perhaps the use of atropin for some time till the muscles 
lose their hypertrophy, will result in a complete cure. 
When the child has already developed a tendency to fix with 
one eye constantly, it is always advisable to compel the use 
of the other for some time each day by a pad or bandage. 
In this way very great improvement in the poorer eye often 
takes place. 

Accurate estimation of the refraction and constant 
wearing of the full correction is a sine qua non of all non- 
operative treatment of squint. Squinting children gen- 
erally have a higher error in the squinting eye than the 
other, and careful refraction places the two on a footing of 
equality and makes possible the improvement of an 
amblyopic eye. If it makes vision sharp and distinct in 
each, it stimulates to the utmost the fusion impulse whether 
the object of regard be near or far. In addition it reestab- 
lishes the normal relation between convergence and accom- 
modation. In many cases it will be found that under 
proper glasses the squint persists when the eyes are used 



284 REFRACTION AND MOTILITY OF THE EYE. 

for near work, showing that the interni are still hyper- 
trophied and unduly active under the slightest accommoda- 
tive stimulus. Such children should be kept under atropin 
for weeks and, if necessary, can have a + 4 D. bifocal 
cemented on their distance glasses for the time as in pres- 
byopia. Many parents will object to putting glasses on 
children at the age of two or three years, but it is much 
preferable to an operation which at this age cannot possibly 




Fig. 100. 

be an intelligent one, though it may result in a cosmetic 
cure. 

It is in such eyes that the exercises with the stereoscope 
or the amblyoscope of Worth are especially helpful, as they 
not only stimulate the fusion but directly exercise the 
weakened extrinsic muscles of the eye. 

This instrument consists of two tubes, each provided 
with a mirror and joined together by a hinge. By means of 
this construction the tubes may be brought together to suit 
a convergence of 60 degrees or separated to suit a divergence 
of 30 degrees. The eye pieces have a focal distance equal to 



HETEROTOPIA. 285 

the distance of the reflected images, which consist of trans- 
parent pictures placed in the grooves at the end of the 
longer tubes. Dr. Black has added to the instrument an 
additional movement in the vertical direction, in order to 
obtain fusion when the lateral deviation is complicated by a 
vertical deviation. This vertical adjustment is effected by 
turning the milled head mounted on the shaft extending 
down from the hinge. In Worth's own words the instru- 
ment is used for the orthoptic treatment of squint in the 
following manner : 

"The child, with his correction on, is held on the sur- 
geon's knees and the amblyoscope roughly adapted to his 
degree of deviation; it is then held before the child's eyes 
and an electric lamp is put in the axis of each tube about 
four feet away. By a simple mechanical arrangement each 
lamp is easily brought nearer to, or put farther away from 
the tube. which it illuminates. A slide showing a cage, for 
instance, is put in the tube before the child's fixing eye, and 
a bird in that before the squinting eye, and the child is told 
what to look for. At first he sees only the cage. The lamp 
before the fixing eye is then taken farther away and that 
before the squinting eye is brought nearer until the child 
sees the bird. By this time he has lost sight of the cage. 
The child is then allowed to grasp the instrument, and, 
assisted by the hands of the surgeon, is taught to vary the 
angle of the instrument so as to make the bird go in and 
out of the cage. Many other similar pairs of slides are 
shown. The average child of 3% or 6 years of age takes a 
very keen interest in the game which he imagines has been 
devised merely for his amusement. Slides which require a 
true blending of the images are then shown. After a time 
it is often found that the angle of the instrument may be 
altered to a very considerable extent, either in convergence 



286 REFRACTION AND MOTILITY OF THE EYE. 

or divergence, while the eyes follow the objects and main- 
tain fusion of the pictures. One often gets a powerful 
'desire' for binocular vision in these young subjects with 
surprising facility. The next step is to equalize the inten- 
sities of the lights. This may usually be done at this stage 
without a return of suppression. In many cases one is 
able to deviate the two halves of the amblyoscope more and 
more at each visit until parallelism of the visual axis is 
obtained/' 

The great disadvantage about all non-operative meth- 
ods is that they take vast amounts of time and patience both 
on the part of the physician and patients, and in many cases 
where parents are of a low order of intelligence it is advis- 
able in the beginning to put the eyes asi nearly as we can 
in a normal position as soon as we discover that careful 
refraction and atropin are not likely to be successful. As 
a rule, however, operations on eyes with good vision should 
be postponed till at the age of six or seven it becomes 
possible to estimate more accurately the muscular condition 
and operate intelligently. This is especially important 
when we stop to think that a child who squints has either 
learned to suppress one image or they are so far apart as 
not to distress him. If now by an ill-considered operation 
we bring those images close together, but without restoring 
a normal muscular balance, we may have obtained a beauti- 
ful cosmetic result but at the cost of putting on the child 
all the muscular strain and headaches that often attend 
latent squints, and that just as he is entering on the ocular 
strain of the schoolroom. From the standpoint of comfort 
and ability to study he was much better off before the 
operation. 

In latent squint we had three sets of tests of the 
extrinsic eye muscles : those which gave the position of the 



HETEROTROPIA. 287 

axes in a state of rest, those which estimated the involuntary 
or fusion power and those which tested the voluntary mus- 
cular power. In actual squint the patient has become so 
accustomed to suppressing one image that we cannot make 
use of the static 1 and fusion tests and must depend entirely 
on version tests with the perimeter or tropometer (see page 
238) and these tests should be carefully and repeatedly ap- 
plied to all possible cases before operation. These tests are 
especially important as enabling us to determine whether 
the muscular imbalance is divided between both eyes, in 
which case it was probably secondary to refractive difficul- 
ties and may disappear with their full correction, or 
whether it is confined to one eye, in which case it is prob- 
ably primary, congenital defect, likely to persist without 
operation. The conditions found are exaggerations of 
those found in latent squint, and the operative treatment is 
along the same lines. 

In binocular convergent squint, even though the 
patient always fixes with the same eye, we invariably find 
an increase of inward rotation associated with a decrease 
of outward, both being about equally divided between the 
two eyes. As a rule these are hyperopic patients who first 
developed an esophoria from over-accommodation. As the 
interni increased, the externi proved unable to resist the 
strain, gave way, and a squint resulted, at first periodic in 
times of fatigue, and later a constant one as soon as the 
patient acquires the habit of suppressing one image. This 
habit is easily acquired in a day or two and explains the 
rapid development of squint, the antecedent latent condi- 
tion being unsuspected. Typical tropometer measurements 
in such cases are: — 



i Duane's test (page 222), however, is a very useful one. 



288 REFRACTION AND MOTILITY OF THE EYE. 

RE. 30 up, 50 down; 60 in, 30 out; 
L.E. 30 up, 50 down; 60 in, 30 out. 
The monocular convergent squints show an entirely 
different set of measurements. In these perhaps one 
internus is congenitally larger or inserted nearer the cornea 
than its fellow in the other eye, or there may be a con- 
genitally defective externus. In such a case equal inner- 
vation will rotate one eye- inward further than its fellow and 
squint appears. But the patient always fixes with the eye 
that sees best and it not infrequently happens that the 
muscular defect is in the fixing eye and not in the squinting 
one. A typical measurement would be: — 
E.E. 30 x 50 50 x 45 
L.E. 30 x 50 65 x 35. 

In this case, if the right eye has the best vision, the left 
would turn in. If the reverse were the case, the right 
would turn in and the patient would be apt to carry his head 
turned a little to the left so that he might look in front and 
still keep his sharpest left eye in its easiest position slightly 
rotated toward the mid-line. Any operation on the appar- 
ently squinting right eye would result in a condition some- 
thing like this : 

E.E. 35 in x 60 out 

L.E. 65 in x 35 out. 

In other words, the axes would now be parallel but the 
patient could turn both eyes much further to the right than 
the left, or a condition of dextrophoria would result which 
would compel the patient to keep his head turned to the 
left in order to see comfortably before him. 

Operation. — After we have determined the muscle or 
muscles at fault, we can decide intelligently which operation 
to do. As a rule the shortening and graduated tenotomies 



HETEROTROPIA. 289 

alone are entirely ineffective in actual squint, and the 
choice lies between their combination, complete tenotomy 
and advancement. 

As a matter of theory the advancement of a weak 
muscle ought always to be accompanied by an equal ten- 
otomy of its antagonist. The tendency of late years is to 
advance one or both externi in preference to tenotomy of 
the interni, which was a few years ago the universal prac- 
tice, but which had the disadvantage of sometimes pro- 
ducing an unsightly sinking of the caruncle and sometimes 
of producing an overeffect. The great advantage of ten- 
otomy over advancement is that it can even in compara- 
tively young children be done under cocain. In an extreme 
binocular convergent squint a strong advancement of both 
externi under ether is advisable and then after a few weeks, 
if the result is not sufficient, partial or complete tenotomy 
under cocain can be used to supplement the original 
operation. 

The same reasoning applies to cases in which the mus- 
cular anomaly is in one eye alone, choice being made of an 
advancement, tenotomy or a combination of the two, 
according to the amount of effect desired. 

In a previous chapter the operations for the shortening 
of a muscle or the exsection of a portion with a subsequent 
suture of the ends have been described and illustrated. A 
still greater effect may be secured by the following opera- 
tion. The conjunctiva is laid back over the insertion of the 
chosen muscle which is caught upon a strabismus hook and 
a sufficient portion excised, the distal end being prevented 
from retracting by being grasped by suitable forceps. A 
silk suture with a needle at either end is then employed, 
one needle being passed through the upper corner of the 
distal end and the other through the lower corner. The 

19 



290 REFRACTION AND MOTILITY OF THE EYE. 

upper suture is then carried beneath the conjunctiva till it 
emerges above the cornea while the lower suture has a 
similar position below the cornea. By traction on these the 
stump of the muscle can, if necessary, be brought clear up 
to the edge of the cornea by undermining the conjunctiva. 
If tied in this position the thread would pass directly over 
the cornea and to avoid this we have an assistant rotate the 
eye to its proper position with forceps and then pass the 




Fig. 101. 

lower suture back through the loop left in the stump of the 
muscle and knot it firmly to the upper suture after drawing 
both reasonably tight. In this way the cornea is avoided 
and the loop acting as a pulley, the traction on the two 
sutures is equalized. The effect of an advancement is some- 
times lost by the sutures pulling out, thus converting the 
operation into a tenotomy and increasing the original de- 
fect. This can be obviated by stretching the opposing 
muscle by several tractions on it with a squint hook, thus 
paralyzing it for a few days and taking the tension off the 
advancement sutures. 

The operation of tenotomy in strabismus can be varied 



HETEROTOPIA. 291 

according to the amount of effect desired. The capsule of 
Tenon is reflected back upon the straight muscles a short 
distance like a sleeve and there are many fibres connecting 
the two. In the so-called graduated tenotomy a small 
opening is made in the conjunctiva over the insertion of 
the muscle and then the central fibres of both muscle and 
capsule are picked up with fine forceps and snipped. This 
causes a very slight effect and one which often disappears 
after healing, because only the central fibres of the muscle 
retract. A greater effect is produced by uncovering the 
insertion of the muscle, making a small opening in the 
capsule at its border, so that a hook can be passed beneath 
it and then cutting the tendon and that portion of the 
capsule which lies directly over it. Much of the capsule 
remains, so that there are many bands which prevent the 
complete retraction of the muscle. 

In the classical tenotomy the blunt-pointed scissors are 
made to undermine the conjunctiva over the capsule clear 
down to the end of the capsular sleeve. A hook introduced 
here raises up both capsule and muscle when it is drawn 
toward the insertion of the muscle and both are cut off. 
The hook is then passed upward and downward under the 
conjunctiva to make sure of getting all the muscular fibres 
since in squint the insertion is often in the shape of a fan. 
When the operation is complete there are few or no bands 
connecting the muscle to the eyeball and the greatest 
possible effect is obtained. Unfortunately the extreme 
retraction often causes an unsightly sinking of the caruncle 
and in some cases the muscle entirely fails to become 
reattached to the eyeball and an actual divergence follows. 
This sometimes occurs several years after operation while 
limited inward rotation of the eye is very common. Thi9 
extreme retraction is sometimes controlled by forcibly 



292 REFRACTION AND MOTILITY OF THE EYE. 

stretching the muscle with the hook before cutting it, 
rotating the eyeball outward till the cornea disappears at 
the external canthus. This paralyzes the muscle for a day 
or two and it does not retract so far. This manoeuvre in- 
creases the temporary effect and lessens the permanent one. 
Too much emphasis cannot be put upon the extreme cau- 
tion which should be used in doing complete tenotomy of 
an internus. Eesection or shortening of the externus with 
only partial tenotomy of its antagonist is much the safer 
plan and is usually sufficient. 

It must not be imagined that we can gauge the effect 
of any operation accurately enough to exactly restore a 
normal balance, though we may hit it fortuitously, but by 
care we can so nearly restore the balance that it can be 
reestablished and maintained by the fusion powers. For 
this reason it is highly important that after any operation 
the full refractive correction be continually worn and that 
more or less exercise of the fusion sense after the method 
of Worth be practised. 

Divergent squints are comparatively rare in young 
people, as they are commonly the result of convergence 
insufficiency from overwork and do not appear till later in 
life. The muscular weakness is generally shared by both 
eyes, typical tropometer record being: — 
K.E. 40 x 55 
L.E. 40 x 55. 

It is very much more difficult to treat these than con- 
vergent squints and in the majority of cases tenotomy of 
one or both externi will be necessary and in many others an 
additional advancement of one or both interni. In some 
cases, however, the tropometer will indicate that the abnor- 
mality is confined to one eye and in such the operative 
treatment should, if possible, be limited to this one also. 



HETEROTROFIA. 293 

Vertical squints are, for the most part, congenital 
anomalies, due either to some asymmetrical development 
of the skull or defect in the origin or insertion of some 
of the elevators or depressors of the eye. The individual 
is often able to compensate for his defect by tilting his 
head till his eyes are on a level, but in many cases where 
there is no attempt at compensation and fusion, the defect 
is not a conspicuous one. We have seen in latent squint 
how slight are the powers of the eye to fuse vertical 
diplopia, and a slight vertical squint by destroying the 
possibility of fusion often makes manifest a convergent or 
divergent error which would have been easily overcome, if 
uncomplicated. Attempts at fusion of vertical diplopia 
being made through the third nerve often result in over- 
innervation of the interni or the ciliary muscle with result- 
ing convergent squint. 

Vertical defects are generally treated by tenotomy, 
preferably of a superior rectus. Vertical strabismus may 
be due to spasm of one or both inferior obliques. In look- 
ing to the left and upward the right eye shoots up more 
than the left, while in looking to the right the left shoots 
up. This is sometimes a true concomitant strabismus but 
in many cases is monocular, the spasm of the oblique re- 
sulting from over-innervation necessitated by the weakness 
or paresis of the synergistic superior rectus of the other eye. 
The treatment is a tenotomy of the oblique through a skin 
incision at the lower inner orbital margin, where its ten- 
don can be picked up. It may be necessary to exsect a bit 
to get sufficient effect, and in the concomitant cases opera- 
tion on both eyes may be needed. 

Operations on the vertical muscles should be done only 
as a last resort, since bad results are very frequent. It 
should always be remembered that these are the cases in 



294 REFRACTION AND MOTILITY OF THE EYE. 

which the use of permanent prisms is likely to be most 
effective, and they should always be tried. 

The treatment of strabismus, when one eye is blind, or 
hopelessly amblyopic is somewhat different, since all we 
can hope to secure is a good cosmetic effect, binocular 
vision being out of the question. In young children, how- 
ever, great pains must be taken to be certain of the hope- 
lessness of the amblyopia by the continued use of bandage 
and atropin in the good eye. The refraction of the good 
eye should be carefully estimated and fully corrected, as, if 
the squint is the result of excessive accommodation, it will 
often disappear from this means alone. 

There is no use in trying stereoscopic exercises or the 
amblyoscope since there is no possibility of developing 
binocular vision (and the same may be said of cases in 
adults which have been present since childhood, since the 
fusion faculty cannot usually be restored after the age of 
five or six years). Neither is there any haste about opera- 
tion after the refraction of the good eye has been corrected 
since the squinting eye is already as amblyopic as possible 
and the cosmetic effect only is sought. I am of the opinion 
that this may just as well be deferred till the child is old 
enough to become self-conscious. By that time very often 
it can be corrected under local anaesthesia. Exact measure- 
ment of the rotation of the squinting eye is often difficult 
owing to the amblyopia present, but it is not important, for 
slight over- or under-correction cannot cause the subjective 
suffering of latent squint when one eye is blind. The good 
eye can and should be measured to see if there is any mus- 
cular abnormality there, but unless this is very marked, the 
amblyopic eye alone should be operated upon. Since 
cosmetic cure and not accurate restoration of balance is 
sought, the simpler the operation the better. Generally a 



HETEROTROPIA. 295 

partial or complete tenotomy of the overacting muscles will 
suffice and tenotomy has the advantage that it can be done 
even in children in a very brief time and under local anaes- 
thesia. 

Atropin immediately after operation causes an appar- 
ent increase in the effect of operation by lessening con- 
vergence. The final result is, however, diminished because 
the cut muscle does not retract so far and therefore has 
more power. 

Tests of Cure. — When one eye is amblyopic, the only 
possible object of operation is cosmetic effect, but when 
good vision is present in both, we are not entitled to con- 
sider a case cured absolutely unless the eyes can pass all 
the tests of muscular balance of the normal eyes. Such 
cares are very rare indeed. One has every reason to be 
satisfied, if he has so nearly restored the balance of the 
muscles that the patient by aid of his fusion power can 
maintain binocular single vision for far and near points 
without obvious discomfort. 

A very convenient way of testing this at the near point 
is to have the patient with both eyes read some Jaeger type 
at the ordinan- distance while the physician interposes a 
pencil midway between the eyes and the page. If the 
patient is fixing the same letters with both eyes, he can 
read without interruption, but if he is using only one eye, 
the pencil cuts off a portion of the text which he cannot see 
without moving his head. 



CHAPTEK XIV. 
OCULAR PARALYSIS. 

We have seen that the movements of the eye are regu- 
lated by centres of different rank. In the first place there 
are the centres situated in the floor of the fourth ventricle 
which govern the action of each individual muscle through 
the oculomotor nerves. A lesion here might involve one 
nucleus and so cause the paralysis of a single muscle or 
several. A lesion lower down, involving the trunk of the 
third nerve, would paralyze the action of all the muscles 
supplied by that nerve, while peripheral lesions still fur- 
ther down might again affect only branches of the nerve 
and so paralyze only single muscles. 

The second group presides over the involuntary func- 
tion of binocular vision, the so-called fusion centres. 
They are involuntary and their localization is not very 
clear. The third group is located in the cerebral cortex, 
and governs the voluntary motions of the eyes up and 
down, to the right and left, etc Lesions involving; one of 
the last named centres do not cause a direct paralysis of 
any muscle, but only prevent the coordination of the two 
eyes or their conjugate motion in some one direction. For 
instance, a lesion affecting the fusion centre might cause a 
paralysis of convergence, apparently affecting both interni, 
but neither one is paralyzed, as can be proven by their turn- 
ing normally to the right or left. A cortical lesion might 
entirely prevent the turning of the eyes to the right, but 
(296) 



OCULAR PARALYSIS. 297 

the right externus and left interims are not individually 
paralyzed, because the eyes can still converge and diverge 
under prism tests. 

Motor Paralysis involves the absolute inability of one 
or more muscles to react to innervation from any source. 
It may be either partial or complete and, when recent, is 
characterized by several distinct symptoms. 

Limitation of motion is the most noticeable, the rota- 
tion of the eye toward the paralyzed muscle being dimin- 
ished or entirely abrogated. For instance, in complete 
paralysis of the right internus there would be an absolute 
inability to turn the eye to the left beyond the mid-line. 
Such a defect would be very noticeable, but if the paralysis 
were incomplete, the defect of motility might be so slight as 
only to be made out with the perimeter or tropometer. If 
we have the patient look at a pencil directly in front, the 
left eye will converge, but the right eye cannot. If we 
carry the pencil to the right out of the field of the paralyzed 
muscle, the eyes resume their normal relation to each other, 
while if we carry it to the left, the right eye lags more and 
more behind and a divergent squint becomes more and more 
apparent. This distinguishes paralytic squint from con- 
comitant which is present in all directions and is unchang- 
ing in degree. 

Primary and Secondary Deviations. — If we have the 
patient fix a pencil directly in front and interpose a screen 
before the paralyzed eye, it will assume the position of 
rest behind the screen and turn slightly outward. This 
deviation in the paralyzed eye is the primary deviation. If 
we now change the screen to a position in front of the 
sound eye and compel the paralyzed eye to fix, it will do so, 
if at all, by an extreme innervation of the weak muscle. 
But since convergence is an associated function of the eyes, 



298 REFRACTION AND MOTILITY OF THE EYE. 

an equal stimulation is sent from the convergence centre to 
the internus of the other eye, and this being perfectly 
healthy undergoes an extreme contraction. Therefore the 
secondary deviation of the sound eye behind the screen is 
much greater than that of the paralyzed eye. If the 
impulse comes from the centre governing conjugate motion 
to the left as when we carry the pencil to the left of the 
mid-line, the over-innervation will go to the left externus 
and the left eye will deviate outward behind the screen. 
This again distinguishes paralytic from concomitant strabis- 
mus in which the primary and secondary deviations are 
always equal. 

False Projection.— When the healthy eye is closed, the 
patient does not see things in their normal position. For 
instance, in the case supposed if he be directed to point 
quickly to some object a little to the left, he will invariably 
point too far to the left, and if told to walk toward it he 
will go in a zig-zag course, starting out too far to the left, 
correcting his mistake and then making it again. The 
object is not correctly localized, because the patient is not 
aware of the actual position of his own eye which is turned 
slightly outward. The image of an object straight in front 
then falls to the outer side of the macula and therefore 
seems to the patient to be located to his left. If the object 
be carried to his left, he makes an effort to follow it with 
the paralyzed muscle. From the amount of energy ex- 
pended he feels as though he had rotated his eye far to the 
left and was looking in that direction, while in reality its 
position is unchanged. In time he learns to form a new 
set of judgments and is no longer troubled by false pro- 
jection. 

Diplopia is present when both eyes are used together 
and turned in the direction of the paralyzed muscle. For 



OCULAR PARALYSIS. 299 

instance in the case supposed, if a candle somewhat to the 
left be regarded with both eves, its image will be at the 
macula of the health}' eye, but in the paralyzed eye it will 
fall somewhat to the outer side of the macula and will 
therefore seem to the patient to be further to the left than 
it really is. He, therefore, sees a sharp distinct candle in 
its normal position and to the left of this another which, 
being non-macnlar, is not so distinct. The further the 
candle is carried to the left, the further from the macula is 
the second image formed and the greater the diplopia, 
while if it is carried to the right, the diplopia diminishes, 
and when out of the field of the paralyzed muscle, dis- 
appears entirely. If the paralysis were incomplete, the 
diplopia and strabismus would not show unless the object 
were carried considerably to the left. 

From the position of the eyes in which strabismus and 
diplopia first appear, from the relation of the images to 
each other and their behavior when the eyes are moved in 
different directions, we diagnosticate which of the ocular 
muscles are paralyzed and how extensively. 

Vertigo. — When the paralyzed eye regards an object 
slightly to the left of the mid-line, it is as previously 
explained falsely localized to the left, and the more it is 
stimulated to turn in this direction the greater the projec- 
tion to the left. Consequently all outside objects appear 
to move with constantly increasing velocity to the left and 
this causes vertigo which the patient soon learns to abolish 
by covering the paralyzed eye or by maintaining an oblique 
position of the head. Our patient by carrying his head 
turned toward the left does not have to use his right 
internus except in looking to the extreme left and therefore 
has no diplopia. Many of these positions of the head are 
characteristic of certain paralyses - . 



300 REFRACTION AND MOTILITY OF THE EYE. 

Old paralyses have many modifications of these symp- 
toms. The patient gradually learns by experience to avoid 
his false projection and again localize objects correctly. He 
gradually learns to suppress- the troublesome image found 
in the squinting eye and no longer complains of diplopia 
or vertigo. Contraction takes place in the antagonist of 
the paralyzed muscle. For instance, in our patient a con- 
traction in the right externus gradually occurs so that the 
eye turns more and more to the right. The squint becomes 
more and more extensive and finally exists not only in the 
field of the paralyzed muscle, but in the entire field of 
rotation. Such cases get to resemble very closely cases of 
concomitant squint and the contraction or hypertrophy may 
remain after the paralysis itself has been cured. 

Varieties of Paralysis. — One single muscle may be 
affected or several. When only one, it is apt to be the 
externus or the superior oblique which have an individual 
nerve supply. Paralysis involving several muscles is most 
apt to be due to lesions involving the motor oculi in some 
or all of its branches. Complete oculomotor paralysis is 
characterized by complete inability to raise the lid, to turn 
the eye in or up or down. Since the externus and superior 
oblique are not affected, the eye is rotated outward and 
somewhat downward by their action. 

The iris and ciliary muscles are paralyzed also so that 
the pupil is widely dilated and immobile, and the accom- 
modation paralyzed. Sometimes the eye protrudes slightly 
because of the relaxation of so many of the extrinsic 
muscles. 

Sometimes all the muscles of one or both eyes are 
paralyzed (Ophthalmoplegia totalis). Again, all the ex- 
trinsic muscles of one or both eyes are involved, the iris 
and ciliary muscles escaping (Ophthalmoplegia externa). 



OCULAR PARALYSIS. 301 

Conversely sometimes the iris and ciliary muscles are 
paralyzed alone (Ophthalmoplegia interna). This may 
occur from involvement of the nuclei for the pupil and 
accommodation, the other nuclei in the brain escaping, or 
from peripheral causes like the use of atropin. 

Etiology. — The lesions causing paralysis, either inflam- 
matory or degenerative, may be situated from the first in 
the nerve tissue, but they are much more commonly the 
result of disease in neighboring structures, such as growths, 
exudates or haemorrhages which cause secondary inflamma- 
tion or compression of the nerves. Syphilis is the most 
frequent, cause of ocular para^sis, whether through perios- 
titis, gummata, exudates or the vascular changes induced 
by it. Another disease which should never be forgotten is 
locomotor ataxia which is very often characterized in the 
early stages by paralysis which may be evanescent. Other 
less frequent causes are toxic conditions, such as lead 
poisoning, diabetes, disseminated sclerosis, or infections 
like tuberculosis. Paralyses are very often the result of 
injury to the skull whether orbital or central. Diphtheria 
occasionally produces paralysis of the iris and ciliary 
muscles, and there is a large group of peripheral paralyses 
which appear to be the result of exposure to cold and are 
hence called rheumatic. Many ocular lesions once con- 
sidered rheumatic are now known to be due to focal infec- 
tions from diseased teeth, tonsils, sinuses and the like. 

Paralysis may be congenital, the most frequent form 
being that of the externus of one or both eyes. In con- 
genital cases contracture of the antagonist does not take 
place and the deformity is only noticeable in movements 
toward the affected muscle. Paralysis or absence of one or 
both superior recti has been noted in cases of congenital 
ptosis.. 



302 REFRACTION AND MOTILITY OF THE EYE. 

Site of the Lesion. — Careful study of ocular paralysis 
offers one of the best means at our command for localizing 
central lesions. 

The diagnosis of an orbital paralysis must be made 
from other symptoms indicating orbital disease. Most 
important are the history of recent traumatism, pain, 
evidences of periostitis or an orbital tumor which can be 
palpated, protrusion of the eyeball, haemorrhage into the 
orbit, etc. Examination of the fundus would sometimes 
show a unilateral optic neuritis from pressure which, if it 
were further back, would affect both eyes. 

Lesions at the base of the brain may be assumed with 
more or less certainty when a number of other cerebral 
nerves are involved gradually in the order of their position 
at the base. For instance an olfactory paralysis would 
argue a lesion in the anterior fossa of the skull. Next in 
order would come the optic nerve and a lesion in this posi- 
tion would be singular in that it would cause complete 
monolateral blindness without changes in the fundus, while 
if it were lower down it would cause optic neuritis, and if 
higher up above the chiasm would cause a hemiopia, 
affecting both eyes. Contiguous nerves would be affected in 
turn, including the ocular nerves. When the trigeminus is 
involved at the base of the brain, neuralgia is a s}rmptom 
which is not observed when the lesion is higher up. 

Fascicular Paralyses are due to lesions of the nerves 
between their nuclei and their emergence at the base of 
the brain. For instance, a lesion in the pedunculus cerebri 
would involve the motor oculi and also the pyramidal tract 
above the point of decussation and therefore cause an 
oculomotor paralysis with a paralysis of one of the extremi- 
ties on the other side of the body. This deduction would 
be a certain one provided the iris and ciliary muscle 



OCULAR PARALYSIS. 303 

escaped, for it would prove that the lesion was high up in 
the peduncle where these fibres are still widely separated 
from the rest of the oculomotor fibres. If the oculomotor 
fibres were all involved with a cross paralysis of an 
extremity, the lesion might conceivably be situated at the 
base and near enough to the peduncle to injure it. 

X u clear Paralyses are due to lesions affecting one or 
more of the nuclei on the floor of the fourth ventricle. 
They appear insidiously, first attacking one nucleus and 
spreading to others in order of their location and affecting 
one or both eyes. 

In cases where all the eye muscles except the ciliary 
and pupillary sphincters are involved, the diagnosis can be 
made with reasonable certainty, since if the lesion were 
lower down and affected the trunk of the nerve, the 
paralysis would be complete. Nuclear paralyses are gen- 
erally due to primary nerve degeneration, the most frequent 
cause being syphilis, but any disease which develops toxins 
such as diphtheria or toxic substances such as lead, alcohol 
and tobacco, may be factors. Tabes, disseminated sclerosis 
and progressive paralysis are not to be forgotten. Nuclear 
paralyses, which are generally multiple, may be single. 
This is especially apt to be the case in tabes, one of the 
evident symptoms of which is ocular paralysis which may 
be slight and temporary or permanent and progressive. 

Diagnosis and Localization of Paralysis is sometimes 
absurdly simple, but often extremely complicated and diffi- 
cult, especially when several muscles are involved. The 
chief complaint of a patient with a recent paralysis is the 
annoying diplopia, and we determine that it is binocular by 
Its disappearing when either eye is covered. The next step 
is to discover which eye has the defective motion which may 
be done in several ways. For instance, if we are using a 



304 REFRACTION AND MOTILITY OF THE EYE. 

candle as a test object, the image in the non-paralyzed eye 
is formed at the macula and is sharp and distinct, while 
that in the non-fixing eye is extramacular and more or 
less hazy and possibly tipped. - Patients commonly speak of 
this without being asked. Then we have them follow with 
the eyes the movement of a pencil in various directions, 
and if the paralysis is complete we shall find that one eye 
lags behind the other in one or more directions. If the 
right eye fails to turn in toward the nose, the internus must 
be at fault; if it lags behind when the gaze is directed 
upward and outward, the superior rectus; if upward and 
inward, the inferior oblique. We examine roughly in this 
way the action of each of the individual muscles, always 
bearing in mind their anatomy and the position of the eye 
in which each develops the greatest power. But in many 
cases the paralysis is not marked enough to be made out 
by such rough methods and we have to base our final diag- 
nosis on the nature of the diplopia and its behavior in dif- 
ferent directions of the gaze. Finally, we can obtain very 
valuable assistance from the measurement of the rotation 
by the tropometer. 

As previously stated, the image in the paralyzed eye 
is fainter and seems to move faster. The distinction is 
sometimes hard to bring out while the patient looks 
straight ahead, but if the object is carried in the direction 
of the greatest diplopia the normal eye will retain its dis- 
tinct image, while the other, being formed further and 
further from the macula, becomes indistinct and seems to 
move faster and faster. (In case of opacities or high re- 
fractive errors the image in the non-paralyzed eye might 
be the more indistinct). To facilitate the identification 
of the false image in further tests we can place a red glass 
over the eye. Suppose we find that the false image is in 



OCULAR PARALYSIS. 305 

the right eye. if the lateral diplopia increases markedly 
when the patient attempts to follow the candle to the right, 
evidently the right externus must he at fault, while if it 
increases when the candle is carried to the left, the right 
internus is defective. If the vertical diplopia increases 
markedly when the gaze is directed upward, one of the 
elevators of the right eye must be paretic, either the 
superior rectus or inferior oblique. It is evident, bearing 
in mind the physiological action of each of these that if 
the superior rectus is the one, the diplopia will be greatest 
when the eye is turned outward so that its optic axis is in 
line with the course of the muscle and then rotated upward, 
while if the inferior oblique is the one at fault, the vertical 
diplopia will be greatest when the eye is turned as far in 
toward the nose as possible and then rotated upward in 
line with the course of the muscle. 

If the vertical diplopia increases on looking down- 
ward, it must be because of the paralysis of one of the de- 
pressors of the right eye, either the inferior rectus or the 
superior oblique. If the inferior rectus, the vertical sep- 
aration will be greatest when the eye is turned outward and 
downward, and if the superior oblique, when the eye is 
turned in and down. 

Eeference has been already made to the projection of 
images which fall on the retina of a squinting eye. If one 
eye fixes correctly while the other turns down, the image in 
the second eye falls below the macula of that eye, and is 
projected upward so that the image is not only fainter, but 
seems higher. If the squinting eye turns in, the image 
is formed to the inner side of the macula and being pro- 
jected outward seems on the same outer side of the mid-line 
as the eye, from which it is called homonymous diplopia. 
If the eye turns out, the image forms to the outer side of 

20 



306 REFRACTION AND MOTILITY OF THE EYE. 

the macula and being projected inward appears on the other 
side of the image of the sound eye (crossed diplopia). In 
other words, the image appears to be opposite to the direc- 
tion of the visual line. If the image isi up, the eye must be 
down ; if the image is to the right, the eye must be directed 
too far to the left, and vice versa. A homonymous diplopia 
always indicates a convergence of visual axes and, if very 
great, a paralysis of an externus. A cross diplopia always 
means a divergence, and if great, a paralyzed internus. 
Another very important fact to be noted is whether both 
images are erect or whether one tips and how much. All 
the muscles of the eye except the internus and externus, 
theoretically cause a certain amount of torsion or wheel 
motion of the eye, and a paralysis of any of them permits 
torsion by the others, with more or less tipping of the 
image in the paralyzed eye which will vary much with the 
direction of the gaze, according to the muscle involved. 
The oblique muscles have much more torsion function than 
the straight ones and where the tipping is extreme a paral- 
ysis of one of them is to be suspected. If the straight 
muscles alone are involved, the tipping is not so great, and 
very often cannot be distinguished at all. 

If we cause the patient to look at a pencil held ver- 
tically in front of him, and study the double images, so 
long as they seem parallel we may practically exclude an 
oblique paralysis. If, however, one of them be distinctly 
out of plumb we can readily tell by interposing a card 
which eye has the tilted image, and if we tip the pencil till 
it seems vertical to the patient its direction must correspond 
to the position of what was the vertical meridian of the 
eye before the paralysis. If this meridian is now tipped 
outward, it must be due to an undue relaxation of the 
superior oblique, if inward, of the inferior oblique. 












OCULAR PARALYSIS. 307 

The same thing will be observed if a Maddox rod be 
placed before the eves alternately with its axis horizontal. 
The sound eye will be conscious of a vertical band of light 
in looking at a candle flame while in the paralyzed one it 
will be slanting until the rod is rotated in the trial frame 
so as to lie at right angles to the true vertical meridian, 
when the line of light will again appear vertical. Knowing 
the position of the true vertical meridian, it is an easy 
matter to decide whether it has been tipped outward by 
paralysis of a superior or inward by that of an inferior. 
The following routine method of studying the diplopia 
will be helpful to the beginner. The ocular images are 
identified by placing the red glass invariably over the right 
eye. bo that the red image always means to the observer the 
right and the white image the left eye. Instead of having 
the patient fix a distant light and get the various eye posi- 
tions by turning his head, the student should face him, 
holding a small flash light for the patient to follow with 
his eyes. In this way he will get a clearer mental picture 
of the muscles called into play. 

Let us suppose a paralysis of the right externus. In 
the primary position there may be a decided lateral dip- 
lopia or it may be very slight. When, however, we move 
the light to the right, the patient's right eye, trying to fol- 
low, lags behind and the red image, instead of being 
formed at the macula, falls on the inner half of the retina 
and gives the impression of being farther to the right than 
it really i-. 

But a paralysis of the left internus also causes a lateral 
diplopia, which also increases as the eyes are turned to 
the right. In this case, however, the left eye lags behind, 
the white image falls on the outer half of the left retina 
and seems further to the right than it really is. In either 



308 REFRACTION AND MOTILITY OF THE EYE. 

case the false image is to the right. To be sure the dip- 
lopia is in one case homonymous and in the other crossed, 
but this is a matter of academic interest only, the impor- 
tant point being that a lateral diplopia, increasing as the 
eyes are turned to the right, indicates a paralysis of the 



RLSup.Rch L 


Inf. Rt.-lnF. 
Ob piques 


L.Sup. Rech 




ml * 


1 Rl\ Intern 




\ / 


L Extern 


RUrrf/Recr. 


5rup\.Rr. Sup 
Obliques 


L.lnf. Rech 



Fig. 101a. 

right hand muscle of the eye having the right hand image. 
Similarly a lateral diplopia increasing eyes left, always 
indicates a paralysis of the left hand muscle of the eye 
having the left hand image. 

The same principle can be applied to vertical diplopias. 
Let us suppose a paralysis of the right superior rectus. 
This will cause, in the primary position, a vertical diplopia 
which is theoretically crossed and with the false image 
slightly tipped. But we have already seen that if there 



OCULAR PARALYSIS. 309 

was a preexistent esophoria, the diplopia may be homony- 
mous, while the tipping is seldom seen and may then be 
due to suggestion or to a preexistent cyclophoria. From 
the diagnostic point they may be dismissed as interesting, 
but not important. If, however, we compel the patient to 
turn his eyes upward either by raising the light or tipping 
his head, the right eye lags in proportion to the amount 
of paralysis, the red image is formed on the lower half of 
the retina and the candle seems higher than it really is, 
while the further the eyes are turned upward the greater 
the vertical separation. The same reasoning holds true 
of the left elevators. Therefore a vertical diplopia which 
increases markedly eyes up, indicates a paralysis of an 
elevator of the eye having the superior image. If the ver- 
tical separation is greatest when the eye is turned slightly 
outward the superior rectus is invoiced, if upward and in- 
ward the inferior oblique. 

If a depressor is paralyzed the affected eye lags behind 
when the light is carried downward, while the (red) image 
falling further and further above the macula seems to move 
rapidly downward. Therefore a vertical diplopia increas- 
ing greatly eyes down, must be due to paralysis of a de- 
pressor of the eye having the lower image, the inferior rec- 
tus if the vertical separation is greatest down and out, the 
superior oblique if down and in. 

The same method can be applied to cases in which 
Several muscles have been paralyzed simultaneously. In 
examining the lateral muscles, the amount of vertical dip- 
lopia should be entirely disregarded and vice versa, while 
it must always be borne in mind that a diplopia which does 
not increase when the candle is carried in the direction of 
the suspected muscle is no indication of paralysis of that 
muscle. 



310 REFRACTION AND MOTILITY OF THE EYE. 

The differential diagnosis is sometimes very difficult: 
— (1) when several muscles in one or both eyes are para- 
lyzed, some completely and some incompletely; (2) when 
there is present a previous heterophoria of some sort. For 
instance a patient with an esophoria sustains a paresis of 
the superior rectus which destroys his fusion power exactly 
as though he were wearing a prism base down before that 
eye. Both eyes at once assume a position of rest, the 
paralyzed eye tending not only downward, but inward, and 
from the position of the double images we should assume a 
paralysis not only of the superior rectus, but a partial one 
of the extemus as well; (3) when the vision in the para- 
lyzed eye is so much better than its fellow that fixation is 
performed by it while the healthy eye is in a state of 
secondary deviation; (4) in old paralysis when secondary 
contractions have taken place or suppression of the false 
image has become a habit. 

In all such cases the tropometer is a most valuable aid 
in the detection of the paralyzed muscle, the measurement 
of the defect, and will show from time to time progress or 
retrogression in the process. 

In measuring the rotatory power of the interni and 
externi the head should be placed in the head rest in the 
usual or primary position (see page 239). In a complete 
external paralysis the eye cannot be brought quite to the 
mid-line and the patient must turn his head slightly to be 
able to fix the object spot at all. The outward rotation will 
be entirely absent while the inward will be unchanged. (It 
may be often apparently reduced because the start is made 
from a point within the field of the internus). In even 
a slight paralysis the rotation is very much more reduced 
than it is ever found in latent squint and by comparison 
with the rotation of the corresponding muscle in the other 



OCULAR PARALYSIS. 311 

eye it is generally possible to distinguish paralytic from 
latent squint with ease. The same tiling is true in par- 
alysis of the interni. The tropometer may be used to 
measure the inferior and superior reefi. The head should 
be placed in the instrument so that the right eye is turned 
slightly outward, and the upward and downward rotation 
carefully measured. The position of the head is now 
changed and the left measured in the same way, being care- 
ful not to change the horizontal plane of the eyes with the 
change of position. Any defects in the vertical rotations in 
these positions must be due to the straight muscles. It is 
impossible to measure separately the elevating and depress- 
ing power of the oblique muscles. 

An exact record of the point and direction where a 
diplopia begins is important to enable us to say later 
whether the process has increased or improved. We place 
the patient at a perimeter and moving a candle or bright 
object along the arc in various directions having the patient 
follow with his eyes, and tell the instant the object appears 
double, we shall have an exact record of his ability to per- 
form binocular fixation in various directions. A still bet- 
ter method consists in having the patient fix the centre of 
the tangent curtain (Fig. 109) with a red glass over his 
right eye, and then follow a small electric light in various 
directions. The points where diplopia begins and the posi- 
tion of the images in each of the principal meridians can 
be marked with chalk or colored pins and charted for 
record. 

We place the patient's head in the primary position 
and have him fix a candle flame at a distance. If he sees 
double images, we place prisms before the squinting eye 
which shall bring the images together. This gives a record 
in prism degrees of the deviation. 



312 REFRACTION AND MOTILITY OF THE EYE. 

The Treatment of ocular paralysis consists in a treat- 
ment of the original cause and also a, symptomatic treat- 
ment to lessen the functional inconvenience. Syphilis 
which is so often the" etiological factor should be combatted 
by very vigorous antisyphilitic treatment. It is to be 
remembered that these are almost invariably tertiary 
deposits calling for very large and increasing doses of 
iodide, and in cases where the etiology is uncertain this 
drug is far more likely to be of benefit than any other. 

The tabetic paralyses/ though probably of specific 
origin, are not benefited by antisyphilitic or any other 
treatment. Fortunately in many cases they are more or 
less evanescent. Paralyses due to haemorrhage should be 
treated by rest and drugs which tend to reduce blood-pres- 
sure, such as aconite, and later by remedies which shall 
cause as much absorption of the clot as possible. The 
iodides in small doses meet both indications and can be 
continued for long periods. 

The toxic paralyses are to be treated by iodide, the 
elimination of toxins being aided by purgatives, diaphor- 
etics and diuretics, while the cases which seem to be 
rheumatic in their etiology do well under the administra- 
tion of salicylates. 

The chief sources of annoyance to the patient in any 
ocular paralysis are the diplopia, false projection and other 
subjective phenomena, which often make it not only 
impossible to work, but make mere existence a torment. To 
remedy this condition, we have two resources, to either blot 
out one image entirely by covering one eye or to bring 
them together by suitable prisms*. Our choice depends on 
the circumstances of the individual case. Prisms are 
especially useful in paralysis of elevator or depressor 
muscles of one eye in which the diplopia is chiefly vertical 



OCULAR PARALYSIS. 313 

and can generally be overcome by a comparatively weak 
glass. It should be just enough to abolish the diplopia 
when the eyes are in the position in which they are ordi- 
narily used, and may be worn over the paralyzed eye or 
divided between the two. In this case the base of the prism 
would be over the paralyzed muscle in one eye, either up or 
down, and in the opposite direction in the other. Of course, 
when the eyes are rotated further in the direction of the 
paralyzed muscle, the diplopia reappears again, since prisms 
can only increase the field of binocular vision. 

In paralysis of an externus a prism or prisms with the 
base out will serve to abolish the diplopia at all distances 
except when the eyes are turned toward the paralyzed side. 
In internus paralysis, on the other hand, the diplopia is 
greater and greater as the object is nearer the eye and a 
very heavy prism would be required with the additional 
disadvantage that the one which abolished diplopia at a 
distance would be useless for near work and vice versa. 

In complicated paralyses or those where very heavy 
prisms would be required, we shall do much better by our 
patient if we abolish the diplopia entirely by excluding the 
paralyzed eye by a pad or better still by a ground glass lens. 
Another procedure is to place over the paralyzed eye a 
prism which shall displace the false image so far up or 
down that it is very easily suppressed. This has the 
advantage of being much less conspicuous than the other 
methods. After a time without treatment of any sort the 
patient either learns to suppress the false image or to carry 
his head in such a position that the diplopia is abolished. 

Operative procedures should be adopted only with the 
greatest caution and nothing should be undertaken till the 
paralysis has certainly reached its maximum of progression 
or of improvement after several months. Tenotomy is 



314 REFRACTION AND MOTILITY OF THE EYE. 

ordinarily not indicated except in old paralyses where con- 
tracture of the antagonist must be overcome, since the 
object to be attained is an increased rotation in the field of 
the paralyzed muscle. This is to be attained only by 
advancement of the muscle, and since it does not affect the 
paralysis but only places the muscle in a more favorable 
mechanical position for using any power it has left, it 
should be considered with great care. 

In paralysis of a right elevator an advancement of the 
superior rectus would place the eye on the same level as its 
fellow in the primary position. There would still be 
diplopia when the gaze was directed far upward, but it 
would not be troublesome in any ordinary occupation, 

In paralysis of an externus an advancement would 
abolish the diplopia straight in front, and it would remain 
only when the gaze was directed toward the defective side. 
A tenotomy of the interims would not be indicated unless 
it was in a state of secondary contracture, since it would 
have no effect on the distance diplopia and would also lessen 
the convergence power of the eyes for near work. 

Complete paralysis of an internus is not ordinarily 
benefited by any operation. An advancement sufficient to 
abolish the X diplopia for distance would be insufficient 
to help that at the near point, while if strong enough to 
accomplish the latter it would overcorrect the former. 
Attempts are made from time to time in paralysis of the 
oblique muscles to make the straight ones assume their 
functions. For instance, in a paralysis of the right 
superior oblique which rotates the eye outward and down- 
ward and intorts it, the insertion of the inferior rectus can 
be advanced outward and upward. In this position it 
depresses the eye and turns it out, while the intorsion can 
be managed by cutting the lower fibres of the internus so 



OCULAR PARALYSIS. 315 

that its traction is chiefly on the upper half of the eyeball. 
Such operations are too problematical in their results to be 
recommended. ' 

Paralysis of Associated Movements. — In all these 
cases there is inability to move both eyes together in a given 
way.' but without any direct paralysis of any ocular muscle. 
The lesion must therefore be situated higher up than the 
nuclei of the ocular nerves, though as yet the centres for 
these different associated movements have not been accu- 
rately localized. 

Paralysis of Convergence. — When complete, there is 
absolute loss of power to converge the eyes beyond the mid- 
line in fixing a near object. At a distance there is a slight 
X diplopia which is more and more marked as the object 
of fixation is brought nearer. There is, however, no 
paralysis of either internus since measurement by the 
tropometer or perimeter will show that they exert their full 
power in rotating the eyes to the right and left, but simply 
do not receive any stimulation from the higher centres when 
convergence is attempted. - These cases are often confused 
with convergence insufficiency, but the distinction is a clear 
one. Paralysis of convergence develops suddenly with 
great subjective discomfort because of diplopia and dizzi- 
ness, false projection, etc. The convergence power is want- 
ing entirely, the patient cannot overcome even a weak prism 
and the diplopia is a constant one, the images being always 
nearly the same distance apart at a given distance. In 
simple insufficiency the development is so gradual that the 
patient seldom complains of diplopia. He can often over- 
come weak prisms and the amount of diplopia is not con- 
stant at all. as after a rest the diplopia may disappear and 
the prism power increase markedly only to diminish again 
with fatigue. The great point in the differential diagnosis 



316 REFRACTION AND MOTILITY OF THE EYE. 

is careful measurement of the rotation of each eye by the 
tropometer. (Inward rotation cannot be measured accu- 
rately by the perimeter). If the abduction and adduction 
are of normal proportions, the diagnosis is established, If 
the adduction is reduced, the diagnosis is at least open to 
suspicion.. 

Paralysis of Divergence begins with a sudden homony- 
mous diplopia, greatest at a distance and becoming regu- 
larly less as the object of regard is brought nearer till 
finally close at hand it disappears. It ordinarily dimin- 
ishes, or at least does not increase when the light is carried 
to either side and the patient can maintain single vision 
much further when the light is carried away from the eyes 
than when it is approached. It is a constant one, not being 
capable of diminution through any effort of the patient 
after rest as in esophoria. Finally there is no limitation 
of the outward, nor increase in the inward rotation of 
either eye as measured by the tropometer, though this limi- 
tation would probably appear secondarily in old paralyses. 

Conjugate Paralyses. — These are paralyses of certain 
voluntary motions of both eyes, together to the right or left 
or up or down without any direct involvement of the ocular 
muscles themselves. They must therefore be caused by 
lesions in the motor areas of the brain. The cortical centre 
for the rotation of both eyes to the right lies in the left 
hemisphere, the innervation proceeding to the nuclei of the 
right externus and left internus. Any lesion affecting this 
cortical centre would prevent the rotation to the right, but 
would not prevent these muscles responding to innervation 
from any other source, as in convergence and divergence. 
In the case supposed the eyes cannot be carried together 
beyond the mid-line by any effort and the position of rest 
is with the eyes deviated toward the left. In other words, 



OCULAR PARALYSIS. 317 

they look toward the lesion. These muscles themselves are 
not paralyzed, because the patient has no diplopia, and 
when the muscles are stimulated from the fusion centres, 
each can overcome the usual prisms showing that neither 
convergence nor divergence is impaired. The rotation of 
the eyes as measured by the tropometer is markedly affected, 
that of the rotators to the right being abolished or greatly 
reduced, and that of the rotators to the left somewhat 
increased. A lesion on the other side of the brain will 
cause a similar paralysis of left lateral rotation and 
similarly the ability to elevate or depress both eyes may be 
impaired. The localization of the lesion in these cases is 
not always simple. Sometimes it occurs in cerebral hemor- 
rhage as a so-called distant symptom through the temporary 
suspension of function of the whole side of the brain, 
though the centre for rotation is not involved. There is 
reason to suppose that the impulse for lateral rotation 
travels through the nucleus of the abducens from which 
a few fibres go to the internal rectus of the opposite side. 
A destructive lesion of a portion of the nucleus might cause 
a conjugate paralysis, but if at all extensive would paralyze 
the externus itself, as shown by inability to overcome prisms 
base in, in that eye. The same conjugate paralysis may 
also be caused by lesion in the pons in both these cases on 
the same side as the paralysis, that is, the eyes look away 
from the lesion. 

Paralysis of upward and downward motion of both 
eyes is said to be caused by a lesion in the quadrigeminal 
region. 

The General Treatment of Associated Paralysis does 
not differ from that of actual muscular paralysis depending 
on a careful study of the probable cause and course. Con- 
jugate paralysis, when dependent on the suspension of 



318 REFRACTION AND MOTILITY OF THE EYE. 

function of a whole hemisphere by cerebral hemorrhage, 
which does not actually involve the associated centre, is 
likely to be recovered from. In conjugate paralysis there 
is no diplopia, but the patient simply complains of inability 
to use his eyes in front and on one side and soon learns to 
compensate by turning his entire head in that direction. 
His comfort in the primary position can often be greatly 
increased by the use of equal prisms over each eye with the 
base in the direction of defective motion. 

The best treatment for convergence paralysis, is an 
exclusion pad and monocular vision. In divergence paraly- 
sis it would seem that advancement of both externi with, 
if necessary, tenotomy of the interni would abolish the 
diplopia at all distances, but if the muscles themselves are 
of normal size, the result would be an insufficiency of con- 
vergence which would cause much more suffering than the 
exclusion of one eye by a pad. 



CHAPTER XV. 
COLOR-BLINDNESS. 

We have seen that when vibrations are set up in the 
ether, they are transmitted in the form of waves of different 
lengths which are perceptible to ns according to their length 
and character, as heat, sound, light, etc. But those which 
excite in us the sensation of light are not all of the same 
length, and, if passed through a prism as in the spectroscope, 
are bent more or less according to their length, so that when 
they are intercepted on a screen, they appear in the colored 
bands of the spectrum. The shortest waves are refracted 
most and seen by themselves give us the sensation of violet. 
As the waves become longer, we get a gradual transition to 
indigo, then to blue, then green, yellow, orange, and finally 
the longest give us the sensation of red. Waves longer than 
these are not manifested as light, but take the form of heat, 
while the shorter ones which are also not perceptible to our 
coarse senses, cause chemical changes as in photography. 
If these colors are again gathered together by appropriate 
means, they give us the sensation of white light. 

Some substances have the power to reflect and absorb 
waves of certain lengths and so give us the sensation of 
color. For instance, a piece of red paper reflects the long 
rays and absorbs the short ones, and so gives us the sensation 
of red. A red glass transmits the red rays and absorbs all 
the others. When a substance reflects all the rays it gives 
us the sensation of white, and when it absorbs all the rays, 
that of black. The character of a color depends on these 
factors, first the wave length of the rays which determines 

(319) 



320 REFRACTION AND MOTILITY OF THE EYE. 

the hue, second the amount of colored light falling on the 
retina in a given time which determines the intensity, and 
third the amount of white light mixed with the color which 
determines the so-called saturation as expressed by the 
words pale, deep or rich. We have seen that all the colors 
of the spectrum when united together make white again, 
but white may also be produced by mixing only two colors 
if properly selected, and any two colors which together make 
white are known as complementary colors, of which the fol- 
lowing with their respective wave lengths are examples : — 

Red X 656 and Blue Green X 492 

Orange X 608 and Blue X 490 

Yellow X 574 and Blue A. 482 

Y r el]ow X 564 and Indigo X 462 

Greenish Yellow X 564 and Violet X 433 

Every color in the red end of the spectrum has a comple- 
mentary color in the blue end, while the complementary 
colors for the middle of the spectrum are composed by a 
mixture of colors from the ends. The complementary color 
for green, for instance, is purple composed of a suitable 
mixture of red and blue, as shown in the diagram. Nat- 
urally, the white formed by the mixture of two colors is 
not so intense as that in which all the colors are combined. 

The results of experiments made by the mixture of 
various pigments do not correspond to those made with 
decomposed white light. The reason for this is that no 
pigments are absolutely "pure," but possess color, because 
when light falls upon them they absorb some rays while 
they reflect others. Thus gamboge reflects the yellow rays 
chiefly, but also many of the green, at the same time absorb- 
ing the blue and some of the red. Thus it is yellow, but 
not the pure yellow of the spectrum. Indigo, on the other 
hand, absorbs the red and yellow and reflects the blue and 



COLOR-BLINDNESS. 



321 






some green rays. When gamboge and indigo are mixed, 
the result is green, because the first absorbs the blue, while 
the last absorbs the yellow and red, while both reflect the 
green. If, however, we take a pure spectral color which 
excites in us the same sensation as gamboge and mix it with 
another which corresponds to indigo, the resulting impres- 




Fig. 102. 

sion is that of white instead of green, which goes to show 
that mixtures of pigment may produce very different effects 
from mixtures of the sensations produced by those pig- 
ments separately. 

Tn dealing objectively with pigments, we find that by 
combinations of the three so-called primary colors red, blue 
and yellow and their derivatives, we can produce all the 
other colors. On the other hand, in dealing with the sub- 
jective sensations produced by decomposed light or com- 

21 



322 REFRACTION AND MOTILITY OF THE EYE. 



paratively pure pigments, we find that by combining in 
various proportions the sensations produced by red, blue 
and green (instead of yellow) we can not only produce 
white, but by varying the proportion and intensity of the 
three, produce the sensation of any other color of the spec- 
trum. These results show that our recognized color sensa- 
tions may be reduced to three, that is to say, our vision is 
trichromic as based on variations of three primary color 
sensations. When, however, we begin to investigate the 
physiological processes by which we apprehend color, we 




Fig. 103. 



JBiu.*- v>our 



are confronted by several rival theories : the chief being 
those of Young-Helmholtz and of Hering. We cannot go 
into these theories deeply, but suffice it to say that most 
facts can be explained by both, but there are a number that 
seem consistent with neither. According to the first theory 
there are three sets of fibres in the retina, all of which are 
excited by every color, but with different intensities. One 
of these is affected most strongly by the long waves and 
gives the sensation of red ; another set which responds most 
strongly to blue waves and a third to the green. A com- 
bination of waves which stimulates more than one set of 
fibres would give a sensation of composite color according to 



COLOR-BLINDNESS. 323 

the amount of stimulation of each as shown iii the diagram, 
which makes no pretense, however, of showing the exact 
proportions of each which cause any sensation. 

For instance, a combination which stimulates the red 
fibres strongly and the green and blue in proper proportions, 
would give us a sensation of orange, and so on through the 
spectrum. A combination which excites all the fibres 
would give the sensation of white light and the absence of 
stimulation would be black. 

If all of these fibres are wanting or not capable of 
stimulation, the individual is totally color-blind. If he 
looks at a colored object, he is only conscious of the white 
light which is present in varying degrees in every color, and 
since the white is faint the object presents instead of color 
various shades of gray as in an engraving. More commonly 
only one set of fibres is wanting, for instance the red. 
When such a person looks at a red object, his red fibres are 
not stimulated and he is conscious only of the stimulation of 
the green fibres and -to a slight extent of the violet, which 
are combined in our sensation of red. A red object makes 
therefore on him the same impression as a green one, but he 
can commonly distinguish between the two by the difference 
in brilliancy. The red object looks green, but since the 
green stimulation is slight the sensation is a feeble one and 
the object looks dark, while a green object which stimulates 
the fibres normally is brilliant in color. In the same way 
there are persons in whom other groups of fibres are absent 
or under-developed and they are green-blind or violet-blind. 

The other theory of color perception is that of Hering 
who supposes three distinct substances in the retina which 
undergo metabolic changes when exposed to light of 
various wave lengths. The first is a red-green visual sub- 
stance which, so long as its metabolism is normal, gives us 



324 REFRACTION A^D MOTILITY OF THE EYE. 

no sensation, but when katabolic changes increase causes a 
sensation of red, and when metabolic changes predominate 
that of green. In the same way there is a yellow-blue 
visual substance which furnishes through katabolic changes 
a blue and through anabolic a yellow sensation. The third 
substance is a white-black which governs our perceptions of 
white and black in a similar way. The two members of 
each pair are therefore not only complementary, but antag- 
onistic. The white-black substance is influenced more or 
less by waves of all lengths, but the others are differently 
influenced by differing wave lengths and by combination of 
sensations from the colors of the spectrum. A person lack- 
ing one or more visual substances entirely or in part would 
be unable to distinguish red and green or yellow and blue, 
or possibly both. According to this theory also, our vision 
is trichromic, for the three pairs play the same part as the 
three primary sensations in the other, and all the results of 
the mixture of colors can be explained on either hypothesis. 

What has been said so far pertains only to vision at 
the macula. As the colored object is moved so that the 
impression falls on the periphery of the retina, it becomes 
fainter and fainter and the extreme periphery may be said 
to be color-blind. The green sensation is lost first, next the 
red, next yellow, and blue last of all. 

It might seem at first sight that color blindness, which 
deprives the eye of one-third of the total retinal stimulation, 
would reduce the visual acuity, but this does not seem to be 
the case, since the general vision of the color-blind seems to 
be as good as that of normal eyes ; when within the range of 
colors which they can perceive, their acuity seems actually 
greater. Even in colors which they cannot see as such, they 
can distinguish differences of shade and tone dependent on 
the admixture of white which are beyond ordinary eyes. 



COLOR-BLINDNESS. 325 

This is to be accounted for on the theory of compensation 
for one defective faculty by compensatory keenness of an 
other. It therefore entails no disadvantage on the subject 
beyond rendering him less fit for the duties of certain call- 
ings which depend largely on the keenness of the color 
sense, such for instance, as the painter, dyer, milliner, etc. 
Since in the railroad and nautical services it is customary to 
use colored signals, chiefly red and green, it is obvious that 
employees who cannot unfailingly distinguish between them 
must be a constant source of danger. Employment in these 
services is therefore in most countries only allowed to those 
who have passed the most careful tests of ability to dis- 
tinguish colors. Such tests should be repeated at infre- 
quent intervals, since while color blindness is generally 
congenital it may also be an acquired condition depending 
on changes in the retina and particularly on atrophy of the 
optic nerve. In this condition color blindness does not set 
in suddenly but gradually the perception for red and green 
diminishes, then for yellow and finally that for blue. In 
retinal diseases and chorioiditis on the other hand the per- 
ception of blue diminishes first. Testing of the color acuity 
may therefore be a very useful aid in the diagnosis between 
diminution of vision by refractive conditions in which it is 
unimpaired and disease of the percipient elements in which 
it is. (See chapter on "Field of Vision"). 

We have seen that few, if any, of the pigments, of 
colored papers, or glasses are pure colors and capable of 
exciting uncomplicated retinal sensations. Consequently 
for the scientific testing of the color sense the spectroscope 
ntial as the only means of giving us pure colors. By 
its air] we can determine whether any portion of the spec- 
trum is wanting and by showing isolated portions of the 
spectrums we can determine the power to name them cor- 



326 REFRACTION AND MOTILITY OF THE EYE. 

rectly and match them with other, similar colors. If the 
patient is affected with red-blindness, the red end of the 
spectrum excites no sensation in the fibres for the perception 
of red and consequently the spectrum is shortened at this 
end. The blue-green of the spectrum seems to him less 
deeply colored than the rest of the spectrum, giving rise to a 




Fig. 104. 



sensation like that of a feeble white or gray. This is spoken 
of as a "neutral band" and it is characteristic of red-blind- 
ness that the band occurs between the blue and the green. 
To the green-blind the spectrum is not shortened, but the 
neutral band falls in the green area, nearer the red end. 

For practical purposes, however, we can approach suffi- 
ciently close to spectral colors with pigments, dyes or 
colored glass, to enable us to test the patient's color capacity 
and to determine in what respect it is defective. In all 



COLOR-BLIXDXESS. 327 

these tests it is important to bear in mind the composition 
of these synthetic colors in order to understand the sensa- 
tion they give a patient who, being more or less color-blind, 
overlooks one or more of the constituent parts and so comes 
to a wrong conclusion regarding the color of the combina- 
tion (Fig. 104). 

There are several methods of ascertaining the ability 
of a patient to distinguish colors of which choice must be 
made according to the circumstances of the case, and great 
care taken neither to deceive one's self nor be deceived. 

For instance, there are persons who have never been 
taught the names of colors and if simply required to name 
colors shown them, will make frequent mistakes on all but 
the simplest colors, while carefully tested by their ability 
to match colors without naming them will show their color 
sense to be normal. On the other hand there are many who 
have an obvious interest in deceiving the examiner, as in the 
examination of railway employees and persons interested 
in litigation for injuries sustained. 

In testing the color sense of railroad employees it is 
only fair that tests should be made which are as much like 
those in the service as possible. For this purpose colored 
glass discs are used revolving in front of a lantern, of such 
size and brilliancy that they shall subtend the same angle 
and have approximately the same brilliancy as the signal 
lamps on the road. The "Williams or Friedenberg test 
lanterns are good types. Xo attempt is necessarily made to 
have the patient name the colors shown, but only to recog- 
nize immediately their significance as indicating a clear 
track, danger, etc. The illumination should be so arranged 
that it can be increased or diminished 1>\ approaching or 
withdrawing the light or by the interposition of different 
shades of smoked or ground glass. 



328 REFRACTION AND MOTILITY OF THE EYE. 

The commonest forms of color blindness are those in 
which red and green are confused and this would be par- 
ticularly dangerous in railway employees, since these are 
the commonest colors employed in signals. A man may be 
color-blind and still be able to distinguish a red light from 




Fig. 105. 

a green in the testing room. The lights do not seem to him 
to diifer materially in color, but he distinguishes at once by 
their varying brilliancy. For instance to the red-blind the 
green signal is bright green, while the red one is the same 
color but very much darker, and he immediately identifies 
it as red simply from its darkness. Evidently, if we show 
him a green light and then, covering it for a second, reduce 






COLOR-BLINDNESS. 329 

the illumination, the color-blind person will get the impres- 
sion of red while normally he should still be conscious of 
green. If he is green-blind;, the green light will appear 
simply as a differently illuminated red, and both these con- 
ditions are often approximated in the service when the 
brilliancy of signals is interfered with by atmospheric con- 
ditions. A man who makes no mistakes in a carefully con- 
ducted test of this kind would be competent enough for the 
ordinary duties of the service, but engineers and signalmen 
should be further tested in a dilferent way to avoid any 
possibility of deceit. The patient is tested according to 
his ability to match colors and especially those which 
experience has proven to be most deceptive to the color- 
blind. This can be done by aid of one of the lantern 
devices which show two lights, one of which is left unchanged 
while the light of the other is colored or changed in 
intensity by the interposition of suitable glass discs, the 
patient being expected to call out rapidly whether the lights 
appear to him similar or not. The same thing can be tested 
by colored papers or powders, but the method in most gen- 
eral use is by the use of the colored wools of Holmgren. 
This has the advantage not only of being convenient, but 
from the very large assortment of different shades of the 
same color, it is possible not only to detect the presence or 
absence of color blindness, but slight differences in the 
acuteness of the color sense as well, which would be very 
important in certain occupations as in dyeing or dressmak- 
ing f >r chemistry. The test is ordinarily performed as fol- 
lows, a good clear daylight being requisite. A skein of pure 
light green is placed by itself and the patient told to select 
from the rest four green Bkeins of darker shade which he 
must do rapidly and accurately. Tf he does this without 
hesitation and correctly, his color sense is probably normal, 



330 



REFRACTION AND MOTILITY OF THE EYE. 



since, if he is either red- or green-blind, he is almost sure 
to pick out some reds which have an intensity similar to the 
green test skein. If he is accurate but slow, his color 
sense must be feeble, while if he makes mistakes even of 
the slightest, he probably has a defective color sense and 




Fig. 106. 



must be further tested to discover which color or colors are 
wanting. For this purpose we set aside for him to match 
a skein of purplish (rose) color (made by the mixture of 
red and blue. See Fig. 104). If he is red-blind, he is 
conscious only of the blue in the skein, while the red 
excites the same sensation as does green. Consequently he 
matches it with dark greens, dark blues or violets. If he is 
blue- or violet-blind — a very rare condition- -he sees only 






COLOR-BLINDNESS. 331 

the red elements in the skein and so matches it with reds or 
oranges according to its luminosity. If he is green-blind 
(green in dyes being composed of a mixture of blue and 
yellow), he is conscious of the blue element in the purple 
(mixture blue and red) and matches it with colors which 
contain blue like the blue-greens or grays (composed of 
mixtures of purple and green). The red element in the 
purple seems very bright to him and exciting the same 
sensation as green leads him to select also very light greens 
which contain an excess of yellow. If he matches colors of 
all kinds, selecting them simply by their intensity or the 
amount of white which each contains, he is totally color- 
blind. 

Briefly stated, the rule is as follows : — 

Test 1. A pure light green, neither a yellow-green nor a 
blue-green, but intermediate: any selection of confusion 
colors indicates at least some color blindness. Slowness 
shows feeble color sense. 

Test II. Purple skein (rose color) : 

1. He who is color-blind by the first test and who upon 
the second selects only purple matches, is not completely 
color-blind. 

2. If he matches the test skein with only blue and 
violet, or one of them, he is completely red-blind. 

3. If he matches it with only green and gray, or one of 
them, he is completely green-blind. 

4. Purple, red and orange indicate violet-blindness. 
Test III. A cherry-red skein of a deep, rich color: If 

he selects as matches for this red, .trreen and brown, which 
to the normal eye appear darker than red, he is red-blind. 
If he selects shades which are lighter than red, he is green- 
blind. 

For the quantitative testing of the color sense, small 



Fig. 107. 



REFRACTION AND MOTILITY OF THE EYE. 



discs of colored paper on 
a background of black 
satin have been devised 
which should be recog- 
nized at definite distances. 
The same effect can be 
obtained with the lan- 
terns previously alluded 
to, the. size of the disc of 
light being determined by 
diaphragms according to 
the distance, or by the 
ingenious instrument of 
Friedenberg which is con- 
structed like a Loring 
ophthalmoscope except 
that the lenses are re- 
placed by" small colored 
discs and there is no 
mirror. These discs can 
be shown at will at the 
aperture, the size of the 
disc and the rapidity of 
exposure being under con- 
trol of the physician. 

Many other tests for 
color blindness have been 
proposed, most of which 
may be useful in doubtful 
cases, but are not in com- 
mon use, and therefore 
have no place in a volume 
of this kind. 




CHAPTER XVI. 
THE FIELD OF VISION. 

We have seen that in the expansion of the optic nerve 
which we call the retina, there is a small region, the macula, 
near the posterior pole of the eye which is much more 
finely organized than the rest, and is the only spot where 
sharp distinct vision is possible. Therefore, when we wish 
to see any object distinctly, we fix it or turn the eye in such 
a way that the image of the object of regard shall fall upon 
the macular region. This is known as central vision, and 
in our studies of visual acuity, refraction, accommodation, 
motility of the eye, and color perception, this has been our 
chief consideration. We shall now proceed to investigate 
peripheral vision which is the function of the remaining 
portion of the retina. Vision here is much less acute, and 
the further the image falls from the macula, the more indis- 
tinct it is as regards form. For perceptions of light and 
movement, however, the peripheral portions of, the retina 
are said to be even more sensitive than the macular. This 
indirect vision is very important to us, since without it, even 
with the moft distinct macular vision the external world 
would make the impression on us of being viewed through 
a double-barreled pea-shooter. We should see nothing 
without looking directly at it and should be constantly fall- 
ing over objects just out of line of the central vision. As 
it is, images falling on the periphery, especially if they are 
in motion, attract our attention at once. We have already 
seen that objects which form an image on any portion of 
the retina are projected in a straight line through the nodal 

(333) 



334 REFRACTION AND MOTILITY OF THE EYE. 

point of the eye, and we know at once that any image which 
falls on the right half of the retina must be situated to the 
left of the eye, and the further to the right the image, the 
further to the left the object. This gives us a correct idea 
of the position of external objects with relation to each 
other. 

When we combine with this the knowledge of our own 
position in space, conferred by our sense of equilibrium, and 
the position of our eyes with regard to our bodies, which is 
conferred by the muscular sensations of the ocular muscles, 
we have a pretty exact estimate of the positions of objects 
in space, both with regard to ourselves and each other. 
Evidently this depends not only on correct retinal percep- 
tion, but on correct cerebral perception and may be inter- 
fered with by abnormalities in either. Careful testing of 
the function of peripheral vision may therefore often be of 
great assistance in determining the existence and location 
of lesions in the central nervous system as well as those of 
the eye itself. All objects which make an impression on 
the retina, whether central or peripheral, are said to be 
in the field of vision, and the examination of this field must 
be made in each eye separately, the other being closed. 
The eye to be examined should be fixed on some object 
directly in front so that it shall remain in the same position 
throughout the examination and should be carefully 
observed from time to time to see that it does not depart 
from this position, otherwise the examination is of no 
value. All tests of the field of vision should be made, if 
possible, in a good clear daylight, with the patient's head in 
such a position that the test object shall have a good and 
uniform illumination. The simplest way of testing the 
field of vision is by using the fingers, a lead pencil or a bit 
of chalk as a test object. The patient is seated close to and 



THE FIELD OF VISION. 



335 



facing the physician. He is directed to close his left eye 
and look directly at the left eye of the observer, who closes 
his right eye. He is thus in a position to make certain that 
the patient keeps his eye constantly in the primary position. 
The test object is then held at a point half way between the 
two and moved slowly up, down, in and out, the patient 





From Peters's " Perimetry." Courtesy of Lea & Febiger 



Fig. 108. 



calling out when it disappears from view. If the object 
is kept constantly in a plane half way between the physician 
and patient, it >hould disappear from view of both at 
exactly the same time, always supposing that the field is 
normal. When the test is made in this way, there is an 
almost unavoidable tendency for the patient to follow with. 
his eye instead of keeping it fixed and it is a much better 
plan to start the object at the periphery out of Bighl of both 
and have the patient call out the instant he is conscious of 



336 REFRACTION AND MOTILITY OF THE EYE. 

its presence, which should coincide with the consciousness of 
the physician. In this test he is not expected to wait till 
he can distinguish the form or the color of the object. The 
same maneuvre is then repeated with the left eye of the 
patient which fixes the now opened right eye of the physi- 
cian. This method is capable of detecting large defects in 
the patient's field and is the only one available when the 
patient's eye is so defective that he cannot perceive small 
test objects. In some cases as in cataract we can arrive at 
valuable conclusions by using, instead of the hand, a 
lighted candle or a small electric light. A great disadvan- 
tage of this method is that while it helps to detect large 
defects, it affords us no means of recording either their 
exact extent or location, so that we cannot, at a later date, 
be certain whether the defect has changed its location or in- 
creased or diminished in size. 

For a more acurate examination which can be re- 
corded, a perimeter is essential. For all practical pur- 
poses this consists of a revolving metal half circle, numbered 
from the centre in degrees each way and so arranged that 
either eye can be placed at the centre of the curvature and 
fix the centre of the arc, the other being covered. 

Care should be taken to have the patient seated with 
his head in the primary position, so that it is turned to 
neither side, nor up or down, and as in the other test, that 
he fixes the centre of the arc constantly. The test object 
should be white and of a definite size (generally one square 
centimetre). In default of this a hat pin with a white 
head makes an excellent test object. Let us suppose that 
we are testing the right eye. We arrange the arc so that it 
is horizontal, to test the field in the meridian of 180° and, 
beginning at .the extreme temporal end of the arc, pass the 
object slowly toward the centre till the patient is conscious 



THE FIELD OF VISION. 337 

of its presence, which is commonly when it is between the 
85th and 90th degrees. Xoting this point, we continue 
along the arc toward the nasal side till the object disappears 
behind the patient's nose. Xaturally this measurement 
depends somewhat on the shape of the nose, but should be 
approximately 60°. Even if the nose were removed, the 
field is not so extensive on the nasal side since the retina 
does not extend as far to the outward side of the fundus as 
to the inward. 

If there have been any places along the arc where the 
test object has disappeared from view or is very dim, we 
note carefully the point of disappearance and emergence. 
For instance, a very small test object will disappear 
momentarily at a point between 10° and 20° to the 
temporal side which will correspond to the blind spot of 
Mariotte opposite the nerve head. Xow we rotate the 
perimetric arc a quarter of a circle or in the axis of 45° 
and 2 2 5° and repeat the procedure, and again in the axes 
90° and 270° and 135° and 315°. If we wish to be 
extremely accurate, we can test an indefinite number of 
meridians. 

For purposes of record the whole can be plotted out on 
a circular chart in which the centre represents the point of 
fixation, the distances in degrees from this point are 
indicated by concentric circles and the different positions 
of the arc are marked in degrees on the circumference. 
(Of course, if both eyes fix the same object and are open, 
the fields of vision overlap each other, most objects directly 
in front coming in both, but a perimetric chart based on 
this would be confusing as it would not show in which eye 
the defective vision was.) One is much less likely to make 
errors of record if he uses a chart like the subjoined which 
shows both eyes together (J?igs. L 13-117). 



338 REFRACTION AND MOTILITY OF THE EYE. 



We can now proceed to test the field of vision for color 
in exactly the same way, substituting for the white object 
green^ red and blue objects, which correspond to the simple 
color perceptions of the retina. In these tests the patient 




Courtesy of G. P. Putnam's Sons 

Fig. 109. 



should not call till he is conscious of the colors and not 
simply the motion of the object, and to be sure he actually 
does this it is better to alternate colors, marking the chart 
with appropriately colored pencils. Below is outlined a 
chart of the right eye showing the normal field for white 



THE FIELD OF VISION. 339 

and the colors. These measurements will all vary some- 
what, depending on conditions present such as the size of 
the pupil which reduces the illumination, the brightness of 
the light, the size of the test object, the projection of the 
nose, cheek bones and eyebrows, and especially the drooping 
of the lids, so that in special cases it is often wise to record 
conditions as well as results. 

lt.fi. 




The perimeter is the best instrument to measure de- 
fects in the periphery but its arc is so close to the eye that 
small but important central defects which subtend a very 
small angle are easily overlooked and poorly recorded. For 
this reason the blackboard or better still the tangent cur- 
tain of Duane give much better tests, since the eye can be 
placed 30 or 60 inches from its centre with a corresponding 
increase in the scale of projection. The degrees marked on 
the curtain are the equivalents of those on the perimeter 
at the distance chosen and can be charted in the same way. 



340 REFRACTION AND MOTILITY OF THE EYE. 

About 15° outward from the fixation point a small 
test object falls on the projection of the optic nerve and 
disappears in the so-called normal blind spot already spoken 
of. If the test object is moved in various directions in this 




Fig. 111. 



area while the patient calls out instantly when it comes into 
view, the spot can be mapped out and marked very accu- 
rately. It seems somewhat larger when the object is moved 
from the blind into the seeing field than in the reverse 
direction, but should be a vertical oval about 7° by 5°. 



THE FIELD OF VISION. 341 

Enlargement of the blind spot is a very important fact. It 
may be due to such manifest causes as a myopic crescent 
but might also indicate such changes in the nerve as occur 
in early neuritis or optic atrophy or incipient glaucoma. 
On the other hand a normal blind spot would exclude many 
pathological conditions. Peters' campimeter is a very 
convenient instrument for rapidly investigating the size of 
the blind spots and central defects (Fig. 111). 

For the discover}* and localization of very small defects 
very small test objects are necessary — used with great care. 

Changes in the field of vision are of two kinds : either 
the boundary of the field is concentrically or irregularly 
reduced, or isolated defects are found within the field 
which are called scot o mat a. The normal blind spot already 
alluded to is a case in point, but they may be present as the 
result of pathological changes. A central scotoma is one 
which involves the point of fixation and therefore greatly 
diminishes, if it does not destroy, the visual acuity, even 
though the limits of the field may not be reduced. The 
so-called peripheral scolomata, if they are distant from the 
point of fixation may never have attracted the patient's 
notice. The mapping of central scotomata is often very 
difficult because the eye being blind so far as its macular 
fibres are concerned, cannot see the fixation mark on the 
perimetric arc and is unable to keep any definite position. 

In these cases the stereoscope may be used with a set 
of the cards devised by Haitz for the purpose. The known 
strength of the lenses and the distance of the cards makes 
it possible to have their scale correspond in degrees to that 
of the perimeter or tangent curtain. Each half of a card 
contains a heavily marked circle or cross which can be 
fused even when one eye has a central defect. With the 
poor eye thus fixed it is possible with the aid of very small 



342 REFRACTION AND MOTILITY OF THE EYE. 



test objects to map out and chart many small central 
scotomata which could otherwise only be suspected. This 
method also offers a very convenient way of mapping out 




Fig. 112. 



the normal blind spots whose average size is indicated on 
one of the cards. The patient should wear his distance 
glasses. This idea has been elaborated by Bausch and 
Lomb in their wide angle stereoscope (Fig. 112). 



THE FIELD OF VISION. 343 

We secure the same end when we have the patient fix 
the white centre of the tangent curtain with both eyes 
open, but with a red glass before the fellow eye. If a green 
test object is used it is plainly visible to the uncovered eye 
but not to its fellow and if a scotoma, is present it disap- 
pears or changes color just as though the other eye was 
occluded. If a green glass is used the test object should 
be red. 

A positive scotoma is one which the patient can see as 
a spot or shadow. It is generally caused by opacities in the 
cornea, lens or vitreous, which coming between a healthy 
retina and the light, cast a shadow and so are perceived 
objectively. A negative scotoma is caused by an actual 
blind area in the retina from which the patient receives no 
impression. In other words, it is one which he cannot see 
objectively, though he may be conscious of its presence as a 
blank space in objects at which he looks, if it is near the 
fixation point. If we have a patient look at a uniform 
white surface like a plastered wall, we can sometimes make 
him conscious of the lack of illumination in this area, which 
then appears as a dark spot. For the time being the nega- 
tive scotoma has become positive. If the retinal area is 
absolutely blind, we speak of the scotoma as absolute, while 
if the sensation is merely diminished, it is said to be 
relative. Eelative scotomata are often only discovered by 
the use of very small test objects and tests with colors are 
very useful in discovering these relative scotomata, since 
colored objects fail to be perceived long before white ones, 
or become suddenly less bright. 

The examination of the field with colored objects is 
therefore a very much more delicate test than with white, 
since it reveal- deficiencies long before they could be 
detected by testing with white. This is especially useful in 



344 REFRACTION AND MOTILITY OF THE EYE. 

the atrophic changes in the optic nerve, in which color per- 
ception is diminished very early. The order in which the 
color sense is lost is also at times helpful. This diminution 
in the red and green perception is suggestive of changes in 
the optic nerve as in retrobulbar neuritis, while diminution 
of the blue sense is more characteristic of retinal and 
chorioidal disease. 






L. E. 



R. E. 




Fig. 113. 



A careful examination of the field of vision is often of 
great diagnostic value, since many diseases, both intra- 
ocular and cerebral, are characterized by charts which are 
so constant as to be almost pathognomic. 

Fig. 113. Beginning glaucoma simplex, where the 
central vision is perfectly normal, the tension not certainly 
elevated and the cupping of the disc not positive, the test- 
ing of the field is most helpful and there is no better means 
of keeping track of the progress of the disease. Blindness 
occurs from a pressure nerve atrophy, and long before there 



THE FIELD OF VISION. 345 

L. E. R. E. 




Fig. 114. 



L. E. 



R. E. 




Fig. 115. 



are any objective changes it is possible to demonstrate an 
enlargement of the blind spot, and later a characteristic 
notching of the field in the upper nasal quadrant. 



346 REFRACTION AND MOTILITY OF THE EYE. 

This narrowing occurs long before there is any reduc- 
tion in central vision and can be detected with a colored 
object long before it is evident with a white one. Eedue- 
tion in the field for colors is always followed by reduction 
for white unless the progress is stopped. 

Pig. 114. Simple atrophy, showing concentric reduc- 



L. E. 



R. E. 




w 



Fig. 116. 



tion, central vision reduced, and the individual becoming 
color-blind early first for red, then green and last for blue. 

Fig. 115. Eetinitis pigmentosa. Central vision may 
be normal in good light. Reduction of field is concentric, 
begins very early and increases enormously when the illum- 
ination is slightly reduced. Blue color sense is lost first. 

Fig. 116. Eight eye shows central scotoma from mac- 
ular chorioidal changes. Similar conditions at times result- 
from high myopia, syphilis, and senile degeneration. The 
field outside is normal for white and colors, but is very 



THE FIELD OF VISION. 



347 



difficult to map out owing to inability of patient to fix and 
keep the eye still with a blind macula. 

Left eye shows typical field in retrobulbar neuritis 
from tobacco, alcohol, etc. Central vision reduced and 
better in dim light. Field for white normal at first, but a 
central scotoma for red and green, and finally for white. 

Fiir. 11T. Eight eve shows sector deficiencv due to 



L. E. 



R. E. 




Fig. 117. 

closure of inferior temporal artery. Left eye : the irregular 
reduction of superior part of field due to gradual separation 
of retina below. It can readily be seen how valuable such 
chart- would be in estimating the progress of the processes 
as well as in the irregular fields caused by retinal hemor- 
rhages, chorioiditis, etc. 

Precedent to the formation of scotoma ta in chorioidal 
exudation, detachment of the retina, subretinal tumors, etc., 
there is very often present a marked distortion of the size 
and shape of external objects. The retina being pushed 



348 REFRACTION AND MOTILITY OF THE EYE. 

forward' often irregularly, the image of an object, for 
instance a vertical line, no longer falls on the exact portion, 
from which the brain gets the sensation of vertically. The 
line, therefore, seems bent more or less, and if the retinal 
displacement is near the point of fixation, the distortion 
may be very great and annoying. Later on the retina loses 
its function and a scotoma develops in the area of distor- 
tion. Occasionally in detachment the retina will become 




Fig. 118. 



reattached and continue its function, and the individual 
with the aid of experience revises his judgment as to the 
meaning of retinal sensation from this area, and things no 
longer seem distorted (Metamorphopsia) . 

Fig. 118 shows a field often found in neurasthenia in 
which owing to fatigue of the retina when continually" 
tested, the field keeps getting gradually and regularly 
smaller, recovering its function after a slight rest. Central 
vision may also show similar evidences of fatigue, but the 
relations between white and the colors is a constant one. 
In hysteria, on the other hand, the field is sometimes 



THE FIELD OF VISION. 



349 



absurdly small without apparently interfering with orienta- 
tion, and irregular and inconstant defects may be present. 




Fig. 119. 



The color fields are often reversed and larger than the fields 
for white. 

It can readily be seen that defects in the visual fields 



350 REFRACTION AND MOTILITY OF THE EYE. 

may not only be due to defects in one or both eyes, prevent- 
ing retinal stimulation, but that even when peripheral parts 
are normal, there may be lesions in the conduction 
apparatus or others further back which prevent their 
reception by the brain itself. For this reason a careful 
study of the diagram which shows the paths of these 
impulses is advisable, 

The retinal fibres unite to form the optic nerve which 
after penetrating the lamina cribrosa, passes through loose 
connective tissue to the apex of the orbit where it emerges 
through the optic foramen. This is really a short bony 
canal, which the nerve fits snugly so that at this point it is 
particularly subject to compression and constriction. 

In the optic groove of the sphenoid the nerve unites 
with its fellow from the other side to form the chiasm. 
Here the optic 'tracts may be said to begin and pass back- 
ward, diverging as they go, toward the subcortical optic 
centres, the external geniculate bodies, the anterior corpora 
quadrigemina and optic thalami. The fibres of the optic 
tracts terminate in cortical ganglion cells in the optical 
area. These are the cells in which retinal stimuli are trans- 
formed into sensations. 

The course of the fibres in the chiasm is also important. 
The fibres from the external half of each eyeball go back to 
the optic tracts on that side, while those from the inner half 
of each undergo a decussation in the chiasm and crossing 
over, join the tract on the other side. There are a few 
which pass from one tract to the other along the posterior 
part of the chiasm which do not reach the eyeballs at all. 
They are not, however, true optic nerve-fibres. The fibres 
which supply the lower part of the retina,, are situated in 
the upper part of the chiasm, and vice versa. Accordingly, 
all objects which are seen on the right of the field of vision, 



THE FIELD OF VISION. 



351 



are perceived through the left half of each retina and the 
left hemisphere and vice versa. Evidently any lesion 
affecting the optic nerve below the chiasm must find its 
effect in partial or complete blindness in that eye, the other 
not being involved at all, unless its nerve is also injured. 
Any lesion of one optic tract above the chiasm must result 



L. E. 



R. E. 




Fig. 120. 

in a symmetrical blindness of the corresponding halves of 
each retina. This is known as hemiopia or hemianopsia. 
If the right tract is destroyed, the right half of eacli retina 
"will not functionate, and the left half of the field of vision 
will be a blank. This hemiopia involving the same side of 
both eyes is known as homonymous, right or left as the 
case may be. We are still further aided in localizing the 
lesion by the reaction of the pupil. A few fibres are given 
off by ea r -h optic nerve which go to the nuclei of the third 
nerve and govern the reaction of the pupil to light. In 
such a case as we have supposed, if a bright light be thrown 



352 REFRACTION AND MOTILITY OF THE EYE. 

on the side of the retina and the pupil reacts normally, we 
know that the retina and the conducting fibres must be in- 
tact, at least as far back as these fibres. The lesion must 
be higher up and involve the perception centres themselves. 
If the pupil fails to react, the lesion must be low enough 
down to cut off these fibres. A lesion involving the chiasm 



L. E. 



R. E. 




Fig. 121, 



will destroy fibres going to the nasal side of each eye and 
cause the outer half of each field to be a blank. This is 
known as bitemporal hemianopsia. The line dividing the 
nasal from the temporal half of each retina as indicated in 
the fields of vision is generally nearly vertical and passing 
nearly through the point of fixation, but since the macula is 
supplied with a special bundle of fibres, the fixation point 
often escapes, the line ot demarkation curving out about it. 
Hemianopsia need not necessarily involve an entire half of 
each eye, since the lesion might destroy only a portion of 
the fibres, but it must necessarilv be svmmetrical. A 



THE FIELD OF VISION. 



353 



central scotoma, of course, points to the involvement of the 
papillo-macular bundle of fibres. It can readily be seen 
how the study of the fields of vision may aid us in localizing 
brain lesions, especially when combined with the informa- 
tion derived from possible paralysis of the internal and 
external ocular muscles. 






L. E. 



R. E. 




Fig. 122. 

Fig. 120. Left homonymous hemianopsia with blind- 
ness of right half of each retina and lesion above chiasm on 
right side. 

Fig. 121. Bi-temporal hemianopsia — inner half of 
each retina blind, lesion in chiasm, almost pathognomic of 
pituitary lesion, as in acromegaly. 

Fig. 122. Binasal hemianopsia, caused by atheroma 
of blood -vessels pressing on right and left of chiasm, but 
Dot involving decussating fibres. 



23 



CHAPTEB XVII. 

THE RELATION OF FUNCTIONAL EYE DISEASES 
TO GENERAL MEDICINE. 

The attention of physicians has been called from time 
to time to the connection of what we rather loosely call 
"eye strain" with various functional and even organic 
defects in other parts of the body. The extremists on one 
side argue that every error of refraction or motility, however 
small, is certain under conditions of continuous use to 
result in an abnormal expenditure of nerve .and muscle 
energy; which even though it does not interfere with the 
keenness of sight is very likely to cause pain or disturb- 
ance of function somewhere. They claim that this modern 
ophthalmology is essentially an American development with 
refmem3nts of diagnosis and treatment that are not prac- 
tised anywhere else in the world, and they present the 
history of case after case of all sorts of conditions ranging 
from simple headache to curvature of the spine which have 
been relieved or cured by appropriate treatment of the eyes. 

The reverse position is taken by many men, including 
most neurologists and not a few ophthalmologists. They 
doubt or entirely deny the theory of reflex causation of dis- 
ease, or at any rate, believe that few serious reflex dis- 
turbances originate in the eyes. They rarely find it neces- 
sary to use cycloplegics, disbelieve in the necessity of exact- 
ness in the correction of refraction and let errors which do 
not result in actual reduction of vision go uncorrected. 
The reasonable position to take is somewhere between the 
two extremes because, while many of the results claimed are 
possibly due to misconceptions of the condition present, to 
(354) 



OCULAR INSUFFICIENCY. 355 

suggestion, to self-deception, or deliberate deception of 
others, there is too much clinical evidence accumulating in 
the records of competent observers to allow the theory to be 
dismissed without a careful examination. 

Definition of Eyestrain. — We say that a patient is 
suffering from eye-strain when his eyes are compelled to do 
work which is beyond their physiological capacity. We 
have no exact standards for measuring the capacity of the 
normal healthy eye, only a standard of averages. We know 
that the average patient can read letters of a certain size at 
a certain distance, but we find many whose acuteness of 
vision is far beyond the average. In the same way we have 
an average ability to focus near objects which is greatest in 
.youth and diminishes regularly with age, but this power 
varies much even in individuals of the same age. We are 
still worse off when it comes to standards for estimating the 
important element of endurance. Small wonder, then, that 
authorities differ widely as to the line which divides the 
physiological from the pathological. It is perfectly pos- 
sible to strain even normal healthy eyes by overuse, and 
this is much more likely to occur under unfavorable condi- 
tions and in lowered health, but the chief strain comes from 
the instinctive effort of the abnormal eye to compensate for 
some optical deficiency by increased muscular exertion. 
The hyperopic and the astigmatic can see distant objects 
clearly only by aid of the ciliary muscles and this effort is 
vastly increased in continuous close work. Likewise in 
binocular vision: the eyes when relaxed may be convergent 
or divergent, but if the extrinsic muscles are powerful 
enough to rectify the alignment easily, the condition can 
hardly be called pathological; while if this requires an 
undue expenditure of energy, eye-strain may be said to be 
present. 



35G REFRACTION AND MOTILITY OF THE EYE. 

We have to deal in each case, therefore, with an 
individual muscular problem, and the determination of the 
exact point where the physiological passes into the patho- 
logical depends not only on a proper estimate of optical 
defects and individual capacity for compensation, but also 
on a careful consideration of a number of variable factors 
such as age, health, and the conditions under which the eyes 
are generally used. Eye-strain is the more easily overlooked 
because it often accompanies perfect vision and, paradoxical 
as it may seem, is much more likely to follow small errors 
than large ones. The patient who has a very large error 
strains for clear vision, but, after a time, ceases because it is 
of no avail, and is content with limited vision, while the 
one with a small error often continues straining because it 
enables him to see distinctly. 

Eyes which are being strained may produce symptoms 
of several different sorts. First and most common are 
those which proceed from muscular fatigue. The ciliary 
muscle, for instance, when tired, ceases to act smoothly and 
vision is sharp and clear at one instant, and the next is 
very indistinct. Then come congestion of the ciliary body, 
with its pains reflected along the nerves of sensation, 
accounting for many a headache and neuralgia. 

Not infrequently, in the effort to supply additional 
stimulation to the tired eye muscles, adjoining muscles are 
innervated and we have twitching of the lids or facial 
muscles. Overstimulation of eye muscles may also deprive 
other muscles of their normal innervation and the same 
reasoning applies to secretion as well. In these ways many 
observers account for the unmistakable stimulation of motor 
or secretory functions of distant organs which are often 
seen in eye-strain cases. 

Perhaps the condition which is most likely to result in 



OCULAR. INSUFFICIENCY. 357 

functional nervous disorders is the cerebral fatigue that 
conies from the constant strain of interpreting retinal 
images distorted by refractive anomalies. When reading, 
for instance, we proceed word by word, and not letter by 
letter; but the astigmatic individual, who easily confuses 
letters, has to pay much closer attention to his text and 
is, in effect, reading proof, which is one of the most 
fatiguing of tasks. To him, all round objects are more or 
less oval and the square ones oblong, and he is under the 
necessity of performing a constant series of mental judg- 
ments as to the actual form of external objects. 

In demonstration of this theory we must not expect the 
absolute proof that would be required in organic diseases of 
germ origin. To be sure, we. can cause some of the con- 
ditions, such as headache, by creating with glasses an 
artificial eye-strain but we must often fail because of the 
enormous variations in individual susceptibility to strain 
and ability to compensate for it. Even the effect of strain 
in the same individual will vary greatly from time to time, 
according to this bodily health and the demands on this 
stock of nerve energy. 

Even from the clinical standpoint we fail of perfect 
demonstration, for the relief of one or two cases is only 
presumptive evidence. A whole series of similar cases 
would be far more valuable, while even a single case in 
which definite symptoms are relieved by eye-glasses and 
recur when they are left off would be entitled to some atten- 
tion if we could more definitely exclude the therapeutic 
influence of suggestion. This element should never be 
forgotten, particularly in the large class of neurasthenics 
and hysterics of whom a great authority has said "if they 
think themselves well they arc well." Such patients who 
have been in a state of depression and despondency, have 



358 REFRACTION AJSD MOTILITY OF THE EYE. 

explained to them a novel theory which not only accounts 
for their symptoms but offers a definite plan of relief, and, 
if they have been at the same time subjected to the mental 
and physical effects of cycloplegia and a routine of impres- 
sive and mysterious instruments, a suggestive therapeutics 
of the most powerful kind is being used. In no other way 
can we account for the occasional happy effects of glasses 
which, through some blunder, really increase the strain they 
were intended to lessen. 

I think we are justified in considering it as a perfectly 
legitimate agent so long as it is not made the excuse for 
careless or defective work, particularly because suggestion 
of this sort is renewed every time the patient puts on or 
takes off his glasses. 

Objective Symptoms. — There are a number of rather 
indeterminate symptoms which frequently indicate eye- 
strain though some of them may also result from other con- 
ditions. In the first place there are certain anatomical con- 
ditions of the skull which must necessarily be accompanied 
by eye-strain, such as inequalities, by which one eye is on a 
higher plane than its fellow or further from the mid-line, 
or further from the object of regard ; or in which the eyes 
are noticeably strabismic. 

There is another set of symptoms marked by screwing 
of the lids in myopia and astigmatism, the elevation or 
depression of an eyebrow, the formation of abnormal 
wrinkles in the brows and at the angles of the eyes, the 
constant blinking of the lids and the tilting of the head in 
unnatural positions in the effort to see distinctly. Many 
chronic inflammatory conditions of the lids and conjunctiva 
are also evidences of congestion from straining; but there 
are many cases in which there are no definite indications to 
show whether a given set of symptoms proceed from the 



OCULAR INSUFFICIENCY. 359 

eye or from some other source and we can only proceed on 
a plan of excluding one organ after another. It is a rather 
curious fact that patients who have objective symptoms of 
one type very often entirely escape those of the other. The 
patient with a blepharitis often shows none of the muscular 
twitchings or disturbances of sensation, such as headaches, 
while the eye-strain neurasthenic often appears, both to 
himself and to others, to have absolutely no ocular abnor- 
mality. 

But there are certain conditions which are so often 
dependent on eye-strain that we are justified in assuming its 
presence in a large proportion of patients, while in others, 
in which the connection is only occasional, the assumption 
should be made much more cautiously. 

HcadacJie. — The reflex symptom which is oftenest the 
demonstrable result of failing ocular compensation is head- 
ache. That this is true is proved not only by the readiness 
with which many persistent headaches yield to proper 
glasses, but also by the ease with which they can be caused 
by improper ones. The great majority of our profession 
have, I think, accepted this view in theory, but in practice 
I doubt if there is any adequate conception of the enormous 
proportion which are so caused. 

Ocular headaches are of two kinds, accommodative and 
muscular. The first usually occurs in individuals who are 
hyperopic or astigmatic but who see perfectly by the aid of 
a ciliary muscle hypertrophied by use. If the strain is too 
great one of the first indications of failing compensation is 
a headache more or less severe that comes on when the eyes 
are steadily used, and is relieved only by longer and longer 
periods of rest. Such headaches are usually frontal and 
vary in character from ?i dull ache to one so constant and 
severe as to cause suspicion of some organic disease. The 



360 REFRACTION AND MOTILITY OF THE EYE. 

ciliary muscle, like other muscles, after prolonged periods of 
inaction, loses any previously acquired hypertrophy and its 
ability to perform work varies with the general bodily 
health. For instance, a patient whose sight has always been 
perfect and painless is confined, or has typhoid, or develops 
some wasting disease. The ciliary muscles fail with the 
other muscles of the body and headaches begin. Such an 
ache is, of course, in a sense the result of illness, but it 
would not occur if the ocular compensation did not fail. 
It can be relieved through the long period of convalescence 
by suitable glasses. In the same way the headache in 
anaemia and disorders of metabolism is very often simply an 
indication of insufficiency of a badly fed ciliary muscle. 

Another type of headache results from imbalance of the 
extrinsic muscles of the eyes. Our eyes may diverge when 
relaxed, but with a good, strong pair of interni we manifest 
no symptoms even when our work calls for continuous con- 
vergence. But if the extra stimulation needed is great, or 
the muscular power is so reduced by ill health that the 
compensation cannct be maintained, the eyes are exhausted 
by the continuous effort to avoid diplopia, and a very annoy- 
ing type of headache ensues. In my experience these pains 
are more aj)t to be referred to the occiput and nape of the 
neck than to the forehead, though accommodative and 
muscular asthenopia so often go hand in hand that the 
distinction is not a clear cut one. 

As a rule an ocular cause may be suspected in all head- 
aches which occur regularly and get worse in the afternoon, 
which are increased by close work and relieved by rest and 
on holidays, or which occur only during certain occupations, 
like the theatre headache or that which comes from travel- 
ing by cars or sightseeing. It would seem that headaches 



OCULAR INSUFFICIENCY. 361 

which occur irregularly and at long intervals were probably 
not due to eye conditions, but there are many exceptions. 

Migraine is a term often misapplied to any severe 
headache, but true migraine is said by neurologists to be a 
kind of sensory epilepsy characterized by an aura, a head- 
ache, and gastric symptoms. The aura is generally a visual 
one taking the form of amblyopia, or in many cases scintil- 
lating scotomata are seen. The pain is almost always a 
hemicrania and may persist for days, the attack being often 
complicated with intense nausea and vomiting. It often 
begins in childhood, is worse during early adult life, and 
declines both in frequency and severity after middle age. 
In many cases it can be traced through several generations 
of the same family. 

There are very many atypical and abortive attacks in 
which one or more of the symptoms are lacking and one 
should be cautious about making a positive diagnosis, but 
many temporary amblyopias, hemianopsias and scintillating 
scotomata, many so called bilious headaches and the like are 
probably related closely to migraine. 

The theory is that a patient inherits or develops a 
state of unstable nervous equilibrium in which reflex invi- 
tation beyond a certain degree excites an explosion. This 
irritation may come from many sources besides the eyes, 
but there is a very large mass of testimony now on record 
a- to cases entirely or largely relieved of their symptoms by 
proper treatment of the eyes. There are, too, many patients 
whose hereditary instability is so great that an explosion 
may occur from any one of several exciting causes. Suit- 
able eye treatment might prevent some of these attacks, but 
not all. 

Epilepsy, — In the same way there is a clinical basis for 
the assumption that eye-strain may be a factor in some 



862 REFRACTION AND MOTILITY OF THE EYE. 

cases of epilepsy. Every such patient has, of course, an 
underlying nervous instability, inherited or acquired, with- 
out which no amount of reflex irritation would cause an 
attack. In the sense of removing the underlying condition 
one would hardly expect a cure but there are many cases on 
record in which undoubted epileptic seizures have entirely 
ceased after suitable eye treatment. The eyes should be 
examined carefully along with the other organs from which 
reflex irritation is most likely to occur. The prognosis is, 
of course, much better in childhood before the attacks have 
become a habit with the patient. 

Chorea is another disease whose dependence on ocular 
insufficiency has been suspected. But there is a wide 
diversity of opinion as to just what chorea is. Even the 
true chorea of Sydenham may not be an etiological unit 
according to many observers. If one considers the disease 
an infectious one he would hardly expect to treat it with 
success through the eyes; on the other hand if one adopts 
the opinion of other good clinicians that chorea is a func- 
tional brain disorder, it is not so difficult to see how it 
might depend on morbid eyes. Until some means of posi- 
tive etiological diagnosis is possible the treatment must be 
more or less an empirical one, and the results various. The 
course of the disease is also so irregular that rapid improve- 
ment might often be in spite of, rather than because of, the 
treatment whatever its nature. The various habit spasms 
which have many resemblances to chorea, but whose nature 
is entirely different, are often amenable to ocular treatment. 

Neurasthenia and Psychasthenia are terms generally 
applied to the symptoms of a class of patients who are 
nervous, irritable, depressed, easily fatigued, physically or 
mentally, and complain of various functional disturbances 
without discoverable organic cause. - 



OCULAR INSUFFICIENCY. 363 

These disturbances are often confined to one organ 
which is considered by patient and physician the seat of 
the disease and we therefore have cerebral, spinal, cardiac or 
sexual types. Some individuals inherit a nervous system 
so irritable that they are from birth unable to cope with the 
ordinary worries of life, but most neurasthenics date their 
breakdown from some illness or shock. More often it is the 
culmination of a long period of overwork or excess. There 
is no factor that plays a more important part in acquired 
neurasthenia especially of the cerebral type than the overuse 
of eyes whose compensation is strained. There is no organ 
in the body where excess is so common and so surely 
disastrous. 

The benefit which neurasthenics often derive from 
rest cures, vacations, and the like, is often, in large part, 
due to the practical relief from all close eye work till com- 
pensation is temporarily re-established. This is one rea- 
son why neurasthenia is so much more common in early 
middle life. 

This is the period which usually determines success or 

failure in life, when work and anxiety are curried to the 

extreme but it is also the time when man's accommodative 

power has finally reached the point where it i> barely sulli- 

cient for his daily needs. Pre byopia normally begins 

n forty and forty-five, but in the hyperopic and 

especially in the astigmatic it may develop much earlier. 

After long periods of strain and symptoms attributed to 

many • - - the accommodation a1 hi t fails bo completely 

• vision is defective, and the eyes receive proper care. 

The last half of life o ten offers the strongest possible con- 

ranquillity and repose. Neurasthenics ooto-* 

riously never die, because many of them never bad any 

trouble that proper glasses would not cure. 



364 REFRACTION AND MOTILITY OF THE EYE. 

Diseases of Metabolism. — We have a long list of dis- 
eases to-day which are generally, and often correctly, attrib- 
uted to faulty metabolism. The popular plan is to treat 
them by careful restriction of diet, entirely losing sight of 
the fact that uricacidemia, indicanuria and the like are 
often simply the expression of functional insufficiency of the 
digestive organs resulting from worry, excesses, or contin- 
uous nervous tension. Some of these conditions are cer- 
tainly due to eye-strain. Such common ills as dizziness, 
nausea, bilious attacks, are often relieved by glasses, while 
the eyes should never be forgotten as possible sources of 
functional digestive diseases. 

The Menopause. — When a woman of any age between 
thirty-five and fifty complains that she has headaches, that 
tilings get dark before her eyes, that she is tired and 
irritable and nervous, it immediately occurs to us that this 
is the period during which sensations of almost any sort 
are to be expected and ascribed to the change of life. 
Curiously enough during this same period presbyopia is 
imminent with its increasing difficulty in reading and sew- 
ing, finally culminating in a frank inability to do longer 
without glasses. Many of the symptoms which women 
resignedly bear for years could be relieved by the oculist. 

Spinal Curvature of the lateral variety is sometimes 
dependent on defective eyes in this way: Owing to orbital 
or muscular abnormalities one eye is on a higher plane 
than the other and to avoid a vertical diplopia the patient 
carries his head tilted more or less to one side and a result- 
ing compensatory lateral scoliosis follows. Gould has sug- 
gested too, that individuals who have a marked astigmatism 
with an oblique axis, get the most dictinct vision of the 
important vertical lines in letters by tilting the head till 
the axis becomes vertical. 



OCULAR INSUFFICIENCY, 365 

Spasmodic Torticollis may follow continuous strained 
postures of the head, resulting from similar optical errors 
and sometimes as a direct result of accommodative spasm. 
The possibility of eye-strain as a factor in conditions of 
which the etiology is so obscure and the treatment so 
unsatisfactory should not be forgotten. 

Differential Diagnosis. — Since most of the functional 
difficulties alluded to may result from other sources of 
irritation beside the eyes, it is often necessary to determine 
as definitely as possible whether the eyes are or are not 
important factors. This can ordinarily be done in two 
ways. If we put the patient under atropine we exclude 
his accommodation, while if we compel him to wear a pad 
over one eye we also temporarily obviate any muscular 
fatigue from the effort to avoid diplopia. 

The cessation of symptoms under these conditions 
would point pretty conclusively to the eyes as the source 
of the trouble and any effect produced by atropine and the 
pad ought to be capable of perpetuation by suitable glasses 
or treatment of the muscular imbalance. The converse is 
not so true, for many cases in which symptoms persist in 
spite of atropine or bandage, yield after a time to suitable 
glasses or an operation. 

The Treatment of eye-strain will frequently tax to the 
utmost the resources of the oculist, but in many individuals 
Nature herself has compensatory powers, and if we can 
bring the error within the limits of those powers we shall 
have given all the relief necessary. This is the reason why 
inexpert work is so often perfectly satisfactory to patients 
in ordinary conditions. In migraine and many other 
nervous conditions, however, it is this very attempl al 
compensation that causes the trouble, and a much closer 
correction i- called for. 



366 REFRACTION AND MOTILITY OF THE EYE. 

In such cases, therefore, it is generally advisable to 
examine the eyes under several days cycloplegia with atro- 
pine, prescribe the full correction for constant wear and if 
there is reason to expect difficulty in accepting so strong a 
glass it is often advisable to keep up the cycloplegia for 
some time. In this way the eyes not only have a period of 
absolute rest, but as they come out from the atropine they 
insensibly become accustomed to their glasses. Neverthe- 
less it must be insisted that patients wear their glasses for a 
time whether agreeable or not and later on if the symptoms 
have subsided, it is possible to sharpen up the distant vision 
by a very slight reduction of the correction. 

The results are likely to vary greatly in the hands of 
different men, first, because many of the cases may turn 
out not to be eye-strain cases at all, and because oculists 
possess many different degrees of skill, judgment and ability 
to handle patients. Finally, good results are to be expected 
only in patients who are intelligent enough to understand 
that the best glass is not necessarily the one which gives the 
sharpest vision. 



CHAPTER XVIII. 
OCULAR MALINGERING. 

The rapid growth of industrial health and accident 
insurance has made malingering a subject of increasing 
importance to the ophthalmologist. There is no industrial 
injury more disastrous to the individual than even a partial 
loss of sight, while there is none so easy to simulate, so 
difficult to detect or making a stronger appeal to commis- 
sion or jury. Mere detection of fraud is not enough, that 
often being regarded as mere human frailty and not penal- 
ized. The chief purpose of the ocular examination is to 
establish a demonstrable estimate of the actual organic and 
functional condition of the eyes. The physician should 
divest himself of bias and seek simply to record actual con- 
ditions, though he feels and reciprocates at once the mental 
antagonism and suspicion of the malingerer as he hesitates 
over each new test in the fear of committing himself. As 
a rule it is a mistake to bully or browbeat The malingerer 
will be more easily detected if he thinks you believe his 
story and are merely making a routine examination for a 
formal record of the facts. 

Detection of malingering does not depend on any one 
or two classical tests but upon an appreciation of the way 
an eye should function under varying conditions, and on 
the ability to put the patient rapidly and naturally into 
positions he does not understand and for which he is not 
prepared. The simpler and more perfunctory the testa the 
better if they throw him off his guard. Above all the be- 
ginner should be perfectly * familiar with a few teste by 

(367) 



368 REFRACTION AND MOTILITY OF THE EYE. 

trying them on himself, and in the more complicated ones 
be sure that he does not adjudge a man guilty because he 
lias himself misunderstood or bungled his tests. A detailed 
record of the examination should be preserved. 

Comparatively few malingerers claim total blindness 
or even partial blindness in both eyes though there will be 
more when the possibilities of toxic retrobulbar neuritis 
become better known. The usual claim is an exaggeration 
of the effect of some accident to one eye or an attempt to 
include in it some pre-existing defect, which may be con- 
genital or acquired and which may or may not affect the 
sight. The reduction of vision to be reasonably expected 
from a low refractive error, a corneal macule, vitreous 
opacities or retinal defects often requires a very nice dis- 
crimination. Refractive defects are by all odds the most 
common causes of reduced vision and the importance of 
giving them their just value and no more is obvious. The 
usual careful objective examination should be made before 
beginning functional tests. 

Inquiry into occupation as showing the visual acuity 
required in every-day life, while a question as to what the 
patient thinks is wrong with his eyes will often give valu- 
able psychological information. The malingerer will in- 
variably start a long story. Examination with a pocket 
flash light will reveal any important external disease, 
opacities in the cornea, the pupillary reactions, and while 
the patient is off his guard enable you to determine, as he 
looks at the light, whether he customarily fixes with one 
eye or both. Retinoscopy, without any cycloplegic, can be 
done by having the patient look off into the distance and 
so relax physiologically. It is of course only approximately 
correct, but it saves time, gives you an idea what the pa- 
tient ought to see without glasses and the kind of lens 



OCULAR MALINGERING. 369 

needed to improve sight Many men can get the same re- 
sult with the ophthalmoscope, while carefully going over 
the media and fundi. In testing the vision it is very im- 
portant to sit and watch the patient rather than the test 
cards as so many men do. If you make him read off 
briskly, keeping him under your eye while he does it, he 
cannot slyly blose one eye. Try to conduct your tests in a 
way he has not been accustomed to, with an appearance of 
carelessness and friendliness. Let him read with both eyes 
open and make him think you are testing his admittedly 
good eye when you are really blocking it out by strong 
glasses. 

Most malingerers are perfectly familiar with the test 
charts but know nothing about the visual angle. Your 
honest man reads at full speed as far as he can and slows 
up only when he ia not sure. The malingerer labors just 
as hard over the big letters as the small ones. In doubtful 
cases make several tests at different distances and see if 
consistent. One of the best means of deceiving the malin- 
gerer is by the use of a mirror in which is reflected a re- 
versed trial card. He is entirely unaware that the mirror 
has increased the distance and reads accordingly. 

The pupillary indications of sight have been consid- 
ered on page 111, a positive direct reaction to light being 
verv good evidence, though not excluding cortical blind- 
Even the Argyl-Kobertson may be unilateral. 

tonally a man claims to he totally blind in one eye, 
even refusing to admit a light which contracts his pupil 
sharply. Often he will not he able to control the redress 
movement in the screen tesl (page 211 ). [nstead of best- 
ing him laboriously with candle and prisms for diplopia, 
it i- much easier to have him fix a hand flash lighl with 
both eyes open, and then interpose a 5-degree prism before 

24 



370 REFRACTION AND MOTILITY OF THE EYE. 

the good eye. If the eye is blind or is not ordinarily used 
in binocular vision it will move in the same direction that 
the good one does as the prism is rotated before the eye, 
while if binocular vision is usual it will not move at all but 
continue to fix the candle. This same method can be 
utilized at the 20 foot distance. If a rotary prism be 
placed before the good eye the distant candle will seem to 
the patient to move evenly toward the apex of the prism if 
the other eye is blind, while if it has sight, even if the 
patient denies diplopia the light will be stationary as* long 
as fusion exists and then seem to jump instead of moving 
evenly. (Lack of binocular fixation is of course no proof 
of blindness.) 

Many methods commonly employed in the testing of 
binocular vision will suggest themselves as useful in par- 
ticular cases, as of course diplopia or the slightest evidence 
of an attempt at fusion will negative a monocular blindness. 
Most trial cases contain the Maddox double prism. If this 
is held before the good eye in such a way that the line be- 
tween the prisms cuts horizontally the centre of the pupil, 
the patient will have a diplopia without knowing that it is 
monocular. If he is honest he will admit it at once while 
if he admits seeing three lights he must be using both eyes. 
If he admits two lights a card should be carelessly held in 
such a way as to cover one of the double prisms and if he 
still sees two lights he must be using both eyes. If he 
looks at a single line of small type he will see three, the 
intermediate one belonging to the uncovered eye. 

The trouble about the c e tests is that while they may 
show fraud and bad faith they give no idea of actual vision 
present. To meet this need we have a whole series of tests 
devised to make the patient think he is using his good eye 
while he is actually using the other. . For the successful 



OCULAR MALINGERING. 371 

use of these it is often necessary to put over the poor eye 
the correction which ought to give it sharp vision while 
over the good one is placed a glass calculated to make its 
vision worse than its fellow. Otherwise the patient can 
tell just which eye he is using by the distinctness with 
which he sees. 

A simple method is to fuss over the bad eye a little 
and apparently give it up as a bad job, but leaving its ap- 
proximate correction in place, and then proceed to test the 
good one with both uncovered. It will very often be pos- 
sible to gradually block out the good eye with a plus ten 
glass before the patient realizes that he is reading with his 
bad eye. 

Have the patient read the test chart while looking 
through the ordinary phorometer. He will see two charts, 
one up and one down, of which he will claim to see one 
much more distinctly than the other. If he is asked to 
read this and then while his attention is distracted the 
prisms are reversed he will often try and be consistent by 
claiming to see plainest the same upper or lower card and 
will read it readily. It is possible to put such lenses in the 
phorometer cells as to make the vision in the good eye less 
than the other and make it difficult for him to know which 
eye he is using if he is prevented from closing one of them. 

One of the best known tests is the so-called red and 
green glass one, in which the patient with a red glass be- 
fore one eye and a green one before the other look- ;ii a 
glass (-hart illuminated from behind with alternate rod and 
green letters etched on it. The red letters arc seen through 
the red glass and the green through the green one. The 
value of the test i> much reduced by the fact that there is 
only one chart on the market which every malingerer has 
seen in the optician's window, while the illumination and 



372 REFRACTION AND MOTILITY OF THE EYE. 

the colors of the glass must be exactly right. It also fails 
to give any definite measure of vision. A much simpler 
test, which would be especially effective on a malingerer 
who had been coached on the foregoing one, is as follows : 
If you put a red glass before the good eye and by having 
the patient look at a red light or a white card establish the 
psychological impression that he ought to see red with 
this eye and not with the other, you can then direct his 
attention to a neighboring card containing alternate red 
and black letters, and ask him to read rapidly. The red 
letters on the white ground cannot be seen through the red 
glass while the black ones are perfectly visible. If he re- 
fuses to read any letters he is of course a malingerer, while 
any red ones seen must be with the alleged bad eye. An- 
other modification of the test consists in placing a green 
glass over the poor eye and the red one over the good and 
have him read from the chart without letting him know 
that there are any colored letters on it. Through the green 
glass the red and black letters look the same color and the 
malingerer does not have his suspicions aroused, while the 
red glass over his good eye prevents his seeing the red let- 
ters with it. "Confusion" letters can be constructed, partly 
black and partly red. Seen through the green glass they ap- 
pear black, while through the red one the red parts disap- 
pear and the letters are entirely different. Red against white 
is invisible through the red glass, but if the background be 
black or dark, or even if the red letter be only outlined 
with a thin black line it becomes visible at once though it 
appears white instead of red. One can easily construct a 
card of red letters, which look superficially just alike and 
yet part of them are absolutely invisible through a red glass 
while the others appear outline letters in white. These 
tests can be used for near as well as distant vision, can be 







T 



E D B D 



Q F E F T 



|A P E O R E K 

Test Types for Monocular Malingering. To illustrat* 
fPlar-f red glass before 'h< good 



80 

7531 

29465 

83 7 56 

2 4 6 8 9 

3 7 5 

Test Types for Monocular Malingering. To illustrate page 372. 
(Place red glass before the good eye.) 



OCULAK MALIXG ERING. 



373 



combined with the various stereoscopic and fusion tests, 
while the advertising pages of any modern magazine will 
supply an infinite variety of letters and labels and pictures 
printed in red. 

The ordinary stereoscope may be most useful, with the 
charts commonly employed in training the fusion, part of 
each picture being seen with each eye. For instance, in 
the chart shown in Fig. 123 the central heavy crosses ap- 



I 2 

3 4 



Fig. 123. 



pear alike to both eyes and are fused into one. If the 
patient is using both eves he sees all four numbers. 2 and -I 
being actually seen with the right eve. Through the stereo- 
scope however, •'!. which is actually seen by the left eve 
appears to be almost directly under the 2, and both being 
on the right of the combined picture seem to be seen with 
the right eye. 

It i- easy enough i<» make op your own card- witb 
print for which the rapid reading requires the use of both 

and the reverse cards which can be read easily writh 
eitber eve alone but not with both together. In using a 

?cope 'jre;it care must be taken that the patient does 



374 REFRACTION AND MOTILITY OF THE EYE. 



not close his pretended bad eye and so deceive you. It is 
better to cut away most of the hood of the instrument. 

The person who claims poor vision in one eye will gen- 
erally admit very good in the other. For this type the bar 
reading test is a good one, having him read fine type with 
both eyes open while you hold a pencil or fountain pen ver- 
tically four or five inches in front of him. If he reads 




Fig. 124. 

without hesitation or twisting of his head he must be using 
both eyes. 

One of the simplest and best means of detecting the 
malingerer is the diploscope. (Fig. 124.) It consists 
essentially of a diaphragm with a central opening through 
which a row of letters is seen with both eyes open. The 
impression is that the letters are all seen with both eyes 
while as a matter of fact those on the left are seen with the 
right eye and vice versa. The fakir who does not under- 
stand the principle involved is very apt to insist that he is 
seeing the right letters with the right eye, which is impos- 



OCULAR MALINGERING. 375 

sible. Unfortunately the device is not practicable for dis- 
tant vision. 

The amblyoscope (Fig. 100) can also be made very 
useful. In all possible positions of the tubes the card in 
the right tube can only be seen with the right eye and vice 




Fig. 125. 



versa but when the tubes are approximated the right card 
as reflected in the mirror appears to be od the Left of the 
other and therefore to be Been with the left eye. When the 
tubes are widely separated the cards are seen homonymously 
while in the intermediate position they may be fused. In 
this last position cards can be used with alternate lines or 
figures on each so thai smooth reading or accurate descrip- 
tion imp]'' -•• of both eyes, while a further source of 



376 REFRACTION AND MOTILITY OF THE EYE. 

confusion can be introduced by having the stronger illumi- 
nation in front of the poorer eye. 

My own "malingeroscope" (Fig. 125) consists of two 
short parallel open tubes the distal ends of which are cov- 
ered by loose caps which can be rotated and which contain 
a small eccentrically placed aperture. By rotating the caps 
the distance between the apertures can be made greater or 
less than the interpupillary distance while being kept in 
the same plane. In the last position for instance, if the 
patient looks at a distant chart with both eyes open he is 
conscious of two holes, the one actually before the right 
eye appearing to the left of the other. If you tell him you 
will cover his alleged bad left eye and slip a card over 
what appears to him as the left hole you are actually cover- 
ing his good right eye and vice versa. If he looks at two 
wall charts 20 feet away and 18 inches apart he can see 
both, the left being seen with the right eye but apparently 
with the left eye through the left hand hole, and vice versa. 
If two exactly similar charts are used many patients will 
fuse them, seeing one hole and one card, and if extra let- 
ters are interpolated at different places on each card will 
read them all as though they were on one. A still further 
source of confusion can be introduced by unobtrusively 
holding a suitable lens over the aperture corresponding to 
the good eye and making its image less distinct than its 
fellow. 

Another series of tests is based on an attempt to make 
an alleged defective eye interfere with the vision in the 
good one. For instance, as the patient reads the wall chart 
with both eyes open, a five degree prism base up is moved 
back and forth before his poor eye. If it is really blind it 
will cause him no inconvenience, while if it has any func- 
tion at all the constant doubling of the test will perceptibly 



OCULAR MALINGERING. 377 

interfere with the smoothness of his reading. The same 
idea can be used in near vision to even better advantage. 
and is even more effective if applied to various projection 
and orientation tests. For instance if you hold up a finger 
ring and ask him to pass a pencil through it or to thread 
a large needle or to catch a ball, the denied diplopia will 
often be very apparent. 

There are cases in which the field of vision is impor- 
tant. For instance policemen, seeking premature retire- 
ment on pension often present all the symptoms of chronic 
retrobulbar neuritis in which there are no very definite 
objective symptoms but a considerable bilateral failure of 
central vision. They are very apt to be unaware that they 
should have a central or paracentral scotoma for red and 
green, with an enlarged blind spot, and generally give a 
field of the hysterical type, extremely contracted,, with the' 
white and color fields nearly equal or even inverted, and 
when tested at different times and distances yielding very 
inconsistent results. They are also very susceptible to sug- 
gestion by the examiner if it is artfully done. In these 
cases too the scotomata should be generally relative and the 
vision ought to vary widely with the brightness of the 
illumination and with the color of the letters used in test- 
ing the central vision. 

Hemiopia seldom suggests itself to the malingerer 
unless he has been previously coached or known some one 
with this defect. Its probability would be enhanced by the 
hemiopic pupillary reaction or some of the other Lesions 
like hemiplegia which so often are associated with it. In 
mapping it- fields the honest patient, will usually show a 
vertical line passing almost through the fixation point, as 
the boundary between the blind and seeing halves of 
retina, and this doc- not change. In testing the main 



378 REFRACTION AND MOTILITY OF THE EYE. 

it is advisable to map out the field on the blackboard or 
curtain, marking plainly its limits, and then after an in- 
terval go over the same ground a second time with the 
point of fixation shifted a few degrees. He is very apt to 
stick to the original limits. The stereoscopic chart shown 
in Fig. 123 is also very useful. In a right hemianopsia, if 
enough central vision remained as is usual the central 
crosses should be fused and seen as one. In actual right 
hemianopsia the figures 2 and 3 could not be seen. The 
malingerer who saw all four would be very apt to assume 
that 2 and 4 were the ones to be denied, if he had carelessly 
been allowed a good look at the card before it was put into 
the stereoscope. In right hemianopsia the honest man 
reads from left to right very badly for he is reading into 
the blind area while he would read letters much faster in 
the other direction. The left hemianope has his difficulty 
in rinding the beginning of the next line. 

If the patient has good central vision in both eyes ad- 
vantage can be taken of the fact that in the diplopia tests, 
either far or near, the sudden interposition of a weak prism 
that throws one image on a really blind half of the retina 
would not cauee any diplopia or fixation movement in 
fusion while in the reverse direction it would. 



INDEX. 



Abducens. 215 

nucleus of. 21.5 

paralysis of. 307 
Abduction, 213 

measurement of. 236, 240 
Aberration, spherical, 42, 104 
Accommodation, 48 

amplitude of, 52, 195 

causing heterophoria, 24b" 

changes in age, 53, 194 

contraction of pupil in, 52, 
111 

cycloplegics in, 53 

in hyperopia, 128 

in myopia, 151 

measurement of, 52 

mechanism of, 51 

nucleus of, 215 

paralysis of. 53, 300 

physiological relaxation of, 
108, 13G 

producing myopia, 147 

range of, 52 

region of, 52, 151 

relation to convergence, 224 

reserve, 195 

spasm of, 140, 145 

simulating myopia., 140, 

145, 154 
treatment of. 118, 140, 154 

-ub-normal, 196 
Accommodative a-thenopia, 131, 
355, 358, 359 

squint. ■J.'I'J 
Acromegaly, field of vision in, 

353 
Acuity of vision, 47 
Adduction, 213 

how measured, 236. 240 
Aerial image, 85, 94 
Age, change- of accommodation 
in, 53, 194 



Albino, luminous pupil in, 

66 
Alcoholic amblyopia, 347 
Alpha (or gamma ) angle, 275 
Alternating strabismus. 275 
Amblyopia, alcoholic, 347 

central origin, 349 

congenital, 274 

ex abusu, 347 

exanopsia, 274 

in >trabismus, 274, 281 
Amblvoscope of Worth, 284, 

1 375 
Amplitude of accommodation, 
oz, 195 

of convergence, 224 
Anabolic retinal changes. 324 
Anaphoria, 243, 268 
Anaesthesia, corneal, 63 
Angle, alpha or gamma, 275 

cr tical, 12 

metre, 224 

minimum visual, 47 

of incidence, 3 

of reflection, 3 

of refraction, 1 1 

refracting of prism, 13 

visual, 46 
Anisocoria, 113 
Anisometropia, 200 

correction of, 202 

fusion weakened in, 201 

retinal images in. 201 

symptoms of. 201 
Anterior chamber, (i l 

in accommodation, 52 
Anterior principal focus, 15 
Aphakia, 204 

diagnosis of. 64 

glasses after cataract opera 
tion. 204. 205 
Apparenl squint, 217. 27 5 

(379) 



380 



INDEX. 



Aqueous humor, 41 
Argyll-Robertson pupil, 115 
Arteries of retina, 75 
Artery, central, 75 
Associated movements, 235 
centres for, 215, 296 
paralysis of, 315 
Asthenopia, accommodative, 131, 
153, 172, 355, 358, 359 
feee Eye-strain. 
cerebral, 201, 357 
muscular, 244, 245 
retinal, 357 
Astigmatism, 62, 163 
acquired, 205 

against the rule, 169, 170; 

corrected by spheres*, 205 

apparent change in disk in, 

174 
asthenopia in, 172 
axis of, 169 
cause of, 168 
charts for testing, 181 
compensation for, 172 
compound hyperopic, 101, 170, 
186 
myopic, 170, 190 
corneal, 164, 165; radius in, 

178 
cycloplegics in low, 192 
effect of on images, 166 

on presbyopia, 197 
estimation of, 83, 98, 101, 173 
images in, 167 
irregular, 163 
lenticular, 164, 167 
low, 192 
mixed, 170, 191 
objective estimation of, 173 

174 
ophthalmometer in, 174 
ophthalmoscope in, 83, 173 
physiological, 164 
produced by cataract extrac- 
tion, 205 
by dislocation of lens, 168 
regular, 165 
retinoscopy in, 98, 101, 173 



Astigmatism, simple hyperopic, 
98, 170, 182 
subjective estimation of, 122, 

182 
symptoms of, 172 
treatment of, 188, 190, 191 
varieties of, 169 
vision in, 167, 171 
with the rule, 169, 170 
Astigmatism, compound hyper- 
opic, 101, 170, 186 
correction of, without atro- 
pine, 186 
with atropine, 187 
general rule for, 188 
Astigmatism, compound myo- 
pic, 170, 190 
correction of, with atropine, 
191 
without atropine, 190 
general rule for, 190 
Astigmatism, simple hyperopic, 
98, 170, 182 
correction of, with atropine 

184 
general rule for, 188 
prescription in, 183, 184 
• symptoms of, 184 
Astigmatism, simple myopic, 
102, 170 
correction of, without atro- 
pine, 189 
general rule for, 190 
sjTnptcms of, 172 
Atrophy of optic nerve, disk 
in, 74 
field of vision in, 346 
scotoma for red in, 344 
Atropine. See Cycloplegics. 
action of, 115 

dangerous in glaucoma, 116 
employed in strabismus, 283 
n refraction, 133. 139, 14?, 
184, 187, 191, 192 
general rule for, 117 
in spasm of accommodation, 

140 
intolerance of, 116 
oily solutions of, 116 



INDEX. 



381 



Atropine, poisoning by, 116 
Axial hypermetronia, 127 

myopia, 145, 156 

ray. 7, 18 
Axis of astigmatism, 169 

of cylinders, 24, 28 

of lens, 19 

of mirror, 7 

optic, 44, 275 

principal and secondary, 19 

visual, 208, 275 

Band of light, 99 
Basal paralyses, 302 
Bifocal lenses, 199 
Binocular diplopia. 206, 217 
single vision, 216 
vision, 206; importance of, 
216; tests of, 217, 295 
Black-white perception, 323 
Blind spot. 338 
Blindness, alcoholic, 347 
brain. 115 
color. 319 
Blue blindness, 331 

in retinal and chorioidal 
disease, 346 
yellow visual substance, 324 

Campimeter. Peters, 340 
Cardinal points, 45 
Cataract, astigmatism after ex- 
traction. 205 
glasses after operation for, 

204. 205 
lenticular opacities in, 70 
myopia in, 145 
refraction after extraction, 

274 
second sight in, 145 
( entreing of glasses, 30, 162 

203, 260 
Centres, nerve, 214, 215 

optical. 19 
fVntrad, 15 

Cerebral asthenopia, 201, 358 
Chiasm, 351 

Chorieform motions from eye- 
strain. 360 



Chorioid, 37 

changes of, in myopia, 148 

functions of, 37 

pigment of, 76 

vessels of, 76 
Chorioidal ring, 74 
Chorioiditis, 148 
Ciliary body, 38 

in accommodation, 51 

in hyperopia, 128 

in myopia, 149 

muscle, 38 

paralysis of, 300 

processes, 38 

spasm of, 145 
Cilio-retinal artery, 75 
Clinoscope of Stevens, 232 
Cocaine with homatropin, 121 
Color blindness, 319 
acquired, 325 
congenital, 325 
in atrophy of optic nerve, 

344 
tests for, 326, 330 
Color perception, theories of, 

3>2, 323 
Color scotoma, 344 
Color-sense, 319 

limits of field, 338 
quantitative tests, 332 
Colors, complementary, 320 

composition of, 326 

confusion, 331 

pigments, 321 

sensations, 322 
Combining lenses, 33, 34, 35 
Complementary colors, 320 
( loncave cylindrical lens, 27 

mirror, 7 

spherical \en&, 22 
Concentric contraction of ficlil 

of vision, 346 
( oncomitant -quint, 270 
( onfusion colors, 331 

letters, 372 
( ongenital amblyopia, 274 

anisometropia, 200 
( onjugate devial ions, 3 16 



382 



INDEX. 



Conjugate motions of eye, 235; 
linear measure of, 236; 
by perimeter, 236; tropo- 
meter, 238 

paralysis, 296, 315 
Consensual reaction of pupil, 

114 
Contraction of field, concentric, 

346; sector shaped, 347 
Contracture in paralysis, 300 
Conus myopic, 150 
Convergence, amplitude of, 224 

capacity for, how tested, 227 

excess of, 254 

insufficiency of, 260 

paralysis of, 215 

prisms, 227 

producing myopia, 147 
Convergent squint, 270 
Convex cylindrical lens, 24 

sphere, 17 
Cornea, 40 

action of, 40 

anaesthesia of, 64 

changes of, in astigmatism, 
164, 165 
in myopia, 144 

measurement of, 154, 174 

opacities of, 63, 70 

radius of, 40, 178; in myopia, 
155; in reduced eye, 45 

reflex* of, 65 

sensitiveness of, how tested, 
64 
Corneal astigmatism, 164, 165 

measurement of, 174 
Corresponding points, 207 
Cortical centres, 215 

paralyses, 296, 315 
Cover test, 217, 222, 225 
Crescent myopic, 150 
Crossed diplopia, 218, 305 

cylinders, 124, 193 
Critical angle, 12 
Crystalline lens, 40 
Curtain, Duane's tangent, 338 
Cyclophoria, 242, 268, 269 
Cycloplegia, when complete, 121 
Cycloplegics, 115. See Atropine. 



Cycloplegics, action of, 116 
causing glaucoma, 116 
choice of, 119, 121 
combined with cocaine, 120 
duration of effect of, 115 
effect on vision of, 116, 121 
final prescription under, 125 
idiosyncrasy against, 116 
indications for, 117 
poisoning from, 116 
static refraction under, 122 
use of, in astigmatism, 184 
190, 192, 193 

in eye-strain, 364 

in hyperopia, 133, 139, 140, 
142 

in myopia, 160, 190 

in "retinoscopy, 88 

in strabismus, 283 
Cyclotropia, 242 
Cylindrical lenses, 24 

action of, 25 

combining, 34 

concave, 24, 27 

cross, 124, 193 

measurement, 31 

recognition of, 31 

uses. See Astigmatism. 

Decentreing lenses, 248 
Decussation of nerve-fibres, 348 
Dennett'si prism nomenclature. 

15 
Deorsumvergence, 231 
Detachment of retina, field in, 
345 
in myopia, 150, 153 
Deviation, angle of, 14 
conjugate, 316 
of eye behind screen, 217, 222, 

225 
primary and secondary, in 
strabismus, 270 
in paralysis, 297 
Dextrophoria, 267 
Diffusion circles, 49, 166 
Dioptre, 30; prism dioptre, 15 
Diphtheria, paralysis from, 301 
Diplopia, binocular, 206, 217 



INDEX. 



383 



Diplopia, character of, in paral- 
ysis. 299 

crossed, 218, 305 

homonymous, 218, 305 

in strabismus. 273 

left, 218 

measurement oi, 310, 311 

monocular, in cataract, 101 

right. 218 

vertical, 305 
Diploscope, 374 
Direct ophthalmoscopy, 69 
Distant vision, 58 
record of, 59 
Disk, ir'lacidos, 175 
Divergence, excess, 261 

function of, 230 

insufficiency, 255 

latent, 242. See Exophoria. 

paralysis. 316 

prism, 230 

vertical, 230 
Divergent squint. 275 
Donders' accommodation table, 

54 
Duane's prism test, 222 

tangent curtain, 338 
Duction, 235 
Dubois in, 115 

Electric ophthalmoscope, 68 
Emmetropia. 62 

ophthalmoscopy in, 78 
retinoscopy in, 92 
Emmetropic eye, 37 

accommodation in. 48, 51, 

53 
amplitude of accommoda- 
tion in. 52 
anatomy of. 37 
axis, optic, 44: visual, 208 
examination of, 55 
far point of, 52, 58 
near point of. 53. 60 
refracting media of. 43 
region of accommodation 

of, 52 
vision in. 48; distant, 58; 
near, 82 



Emmetropic eye, visual angle 

in, 46 
Epilepsy from eye-strain, 361 
Equilibrium, position of. See 

Position of rest. 
Eserin, action of. 125 

as ciliary tonic, 126 
Esophoria,*242. 253. See Ectcr- 
ophoria. 

accommodative, 254 

convergence excess, 254 

divergence insufficiency. 255 

measurement of, 254 

monocular, .257 
Esotropia, 242. See Convergent 

strabis7nus, 270 
Examination of patient, 54 

of anterior chamber, 64 

of cornea, 63 

of eyes, 56 

of field of vision, 333 

of iris, 64 

of lens, 65 

of media, 69 

of pupil, 64 
Excavanon, atrophic, 74 

glaucomatous, 74 

physiological, 74 
Exophoria, 242, 258. See Eeter- 
ophoria. 

accomodative, 259 

convergence insufficiency, 200 

divergence excess, 261 

monocular, 262 
Exotropia, 242. See Divergent 

strabismus. 
Extorsion, 212, 213 
Extrinsic muscles, 210 
Eye, anatomy of, 37 

emmei ropic eye, :>>~ 

hygiene of in myopia, 150 

ideal, 37 

opt ical properties of, 43 

reduced, 15 

schematic, 105 

See lathenopia. 

anatomical causei of, 358 

cerebral fatigue from, 358 

definition of. 355 



384 



INDEX. 



Eye-strain, diagnosis of, 365 
distant organs affected by, 

356 
epilepsy from, 361 
headache in, 359 
in astigmatism, 172 
in hyperopia, 131 
lacking in high hyperopia, 

138 
in pure myopia, 153 
metabolism affected by, 364 
migraine from, 341 
neurasthenia from, 362 
objective symptoms of, 358 
pains from, 356 
scoliosis from, 364 
subjective symptoms, 355 
suggestion in, 357 
torticollis from, 365 

False image in paralysis, 298 
Far point, 52 

determination of, 52 
in emmetropia, 52, 58 
in hyperopia, 151 
in myopia, 151 
recession of in age, 53 
Far sight, 62, 127. See Hyper- 
opia. 
Field of fixation, 311 
binocular, 311 
measurement of by per- 
imeter, 236 
monocular, 236 
Field of vision, 335. See Sco- 
toma and Hemianopia. 
confrontation test, 335 
extent of, 337 
for colors, 339 
in acromegaly, 353 
in amblyopia toxica, 347 
in atrophy, 346 
in brain lesions, 353 
in glaucoma, 347 
in hysteria, 347 
in neurasthenia, 348 
in retinal detachment, 347 



Field of vision in retinitis pig- 
mentosa, 346 
in retro-bulbar neuritis, 

347 
in tobacco amblyopia, 347 
measurement of, 335 
perimeter test, 336 
projection of, 333 
recording of, 337 
Focal distance, principal, 45; 
illumination, 62; length 
of mirror, 7 
Foci conjugate, 20 
principal, 19, 45 
secondary, 19 
virtual, 23 
Focus of mirror, 7 

of lens, 18, 20 
"Fogging," 138 
Fovea centralis, 40 
Friedenberg's color test, 332 
Fukala operation for myopia, 

161, 204 
Functional eye diseases and 

general medicine, 354 
Fusion, 219. See Heterophoria. 
Exophoria, Esophoria, Hy- 
perphoria, Strabismus. 
centres, 215 
deorsumvergence, 23 1 
dependent on, 201, 240 
divergence, 230 
education of, 247, 284 
in anisometropia, 201 
in strabismus,, 277, 279, 281 
relation of fusion powers, 

233 
sursumvergence, 230 
tests, 227, 229, 230, 231 
torsion, 231 

Glasses, 16. See Lenses. 
bifocal, 199 

centreing of, 30, 203, 260 
concave, 22 
convex, 17 
cylindrical, 24, 27 
effect on apparent size, 202 
for astigmatism, 188. 190 



IXDEX. 



3S3 



Glasses, for hyperopia, 133, 142 
for myopia, 156 
for presbyopia, 197 
for strabismus. 283 
Franklinic, 199 
minus, 22 
plus, 17 

prescription for, 35 
prismatic effect of, 203 
prisms, 13 
spherical, 17 

strength varying with dis- 
tance from eye, 159, 205 
Glaucoma., corneal anaesthesia 
in, 64 
field of vision in, 344 
Graefe equilibrium test, 226 
Green blindness, 323, 331 

scotoma for, in nerve dis- 
ease, 344 

Hair optometer, 61 
Headache, accommodative, 359 

from asthenopia, 359, 361 

from astigmatism, 184 

from hyperopia, 131 

muscular, 360 

test of ocular cause, 360, 365 
Hemianopia, 353. See Scotoma. 

bi nasal. 355 

bitemporal, 354 

homonymous, 353 
Heterophoria, 242. See Cyolo- 
pnona, Lsophoria, Exo- 
y ho r ia . // y pe rp h oria. 

advancement in, 289 

causes of. 243 

education of fusion in. 247 

operations for. 249, 252, 289 

prism exercise in, 247 

prisms for constant use, 248 

refraction in. 246 

short filing of muscles, 252 

symptoms, 244 

tenotomy for, 249 

treatment, 246 
Heterotropia, 270. Spp Strabis- 
mus. 



Holmgren test for color blind- 
ness, 329 
Homatropin, 115, 119, 120, 121. 

See Cycloplcgics. 
Homonymous diplopia, 218, 305 

hemianopia, 351 
Hyaloid membrane, 41 
Hyoscyamin, 115 
Hyperopia, 62, 127 
absolute, 129 
after cataract, 204 
asthenopia in, 130, 196 
axial, 127 
causes of, 127 
causing early presbyopia, 130, 

196 
ciliary muscle in, 128 
concealed by accommodation,, 

129 
correction by convex lenses, 

128, 136, 141 
cycloplegics in, 134, 139, 140 

142 
developing in old age, 198 
diagnosis, 132 
eyeball in, 128 
eye-strain in, 130 
facultative, 129 
far point in, 151 
latent, 129 
lenticular, 127 
manifest, 129 
near point, 130, 151 
objective estimate, 134 
ophthalmoscopy in, 79 
prescription in, 141, 142 
producing strabismus, 132, 

278 
refractive, 127 
region of accommodation in. 

151 
retinoscopy in, 93, 97, 134, 

135, 138 
simulating myopia, 138, 1 10 
subjective tests, 136, 138 
symptoms, 12s. ]:;o. See 

Asttu nopia, 
treatment, 133, I 12 
rision in. [28, 130 



386 



INDEX. 



Hyperopia astigmatism, 170, 
182, 186. See Hyperopia 
and Astigmatism. 
estimation by ophthalmo- 
scope, 83 
by retinoscopy, 99, 101 
symptom^ 172, 184, 355 
Hyperphoria, 242, 263. See 
Heterophoria. 
abolishing fusion power, 263 
equilibrium tests, 264; their 

importance, 266 
fusion tests in, 264 
treatment by prisms, 266 

by operation, 266 
tropometer measurement of, 
265 
Hypertropia, 242. See Strabis- 
mus. 
Hypophoria, 242. See Hyper- 
phoria. 
Hypotropia, 242 
Hysterical anaesthesia of cornea, 
64 
field of vision, 348 

Ideal eye, 37 

Identical pointsi, 207 

Illumination focal, lateral, ob- 
lique, 63 

Image, apparent or false, 279, 
304 
in astigmatism, 167 
of lens, 21 
of mirrors, 6, 8, 9 
real or true, 279, 304 
reflex of Purkinje Sanson, 64 
retinal, 46; size of, 46 
tipped, 306 
virtual (of lens), 22, 23 

Imbalance, muscular. See Het- 
erophoria. 

Inch system of numbering 
lenses, 29 

Inclined double images, 306 

Index of refraction, 11, 43 

Indirect ophthalmoscopy, 84 



Insufficiency of convergence, 

260. See Exophoria, 242 

of divergence, 255. See Eso- 

phoria, 242 
of ocular muscles. See Heter- 
ophoria, 242 
of vertical muscles. See Hy- 
perphoria, 242, 263 
Internal rectus, 210 
Intorsion, 209 
Intrinsic muscles, 52 
Inversion, lateral, 6 
Inverted image, 84 

parallactic displacement with, 
87 
Iris, 42. See Pupil, 111 
examination of, 42 
function of, 42 
retinal pigment of, 42 
sphincter of, 42 
Irregular astigmatism, 163 
reflection, 6 

Jaeger's test types, 60 
Javal and Schiotz ophthalmom- 
eter, 1/5; error of, 180 

Katabolic retinal changes, 324 
Kataphoria, 243, 268 
Katral lenses, 36 
Keratometer, 175 
Keratoscope, 175 
Kindergarten test cards, 57 

Lsevophoria, 267. See Hetero- 
phoria, 242 
Lamina cribrosa, 73, 350 
Lantern tests for color sense, 

327 
Latent hyperopia, 129 

squint, 242 
Lateral hemianopia, 351 
illumination, 63 
inversion, 6, 58 
Lens, crystalline, 40, 50 
capsule of, 41 

changes of in astigmatism, 
164, 167 
in accommodation, 51 
in hyperopia, 127 



INDEX. 



387 



Lens, changes of in myopia, 1-45 

cortex of, 41 

effect of zonula on, 51 

measure, 33 

nucleus of, 41 

opacities in, 70 

removal of. for myopia, 161 
Lens rack, 110 
Lenses, 16. See Glasses. 

axial ray of. 17 

axis, principal and secondary 
of, 19 

centreing of ; 30, 203, 260 

clerical, 201 

combinations of, 33 

concave, 22 

conjugate foci of. 20 

convex, 17 

cylindrical convex, 24; con- 
cave. 27: axis of, 24, 26, 
28 

decentreing of, 248 

focal length of, 20 

focus of, IS 

image, real. 21: virtual, 22, 
23 

katral, 36 

measurement of, 29, 31, 33 

nodal point of, 19 

numbering of. 29 

optic centre of, 30 

peri scop ic, 36 

principal axis of. 19 

prismatic effect of. 162, 203. 
24^. 260 

prisms. 13 

recognition of, 31 

spherical aberration of, 18 

toric, ••;•; 
Light, theory of, 1 

perception of. how tested, 330 

reaction of pupil to, 114 
Limited motion in paralysis, 

after squint operation, 291 
Linear measurement of motil- 
ity. 271 



Locomotor ataxia, paralvses in, 
301 
Ar£vll-Robertson pupil in, 
"llo 
Loring ophthalmoscope, 66, 69 
Luminosity of pupil, 65 

Macula lutea, 39, 76 

changes of in myopia, 149 
Maddox double prism, 224; rod, 
221 
in oblique paralyses, 307 
Ma lingering, amblvoscope in, 
375 

color tests in. 371 

confusion letters in, 372 

diplopia tests in, 370, 371 

diploscope in, 374 

exclusion tests in, 371 

malingeroscope in, 376 

mirror tests in, 369 

ocular, 367 

prism tc<ts in, 370, 371, 376 

pupils in, 369 

refraction in, 368 

stereoscope in, 373 
Malingeroscope, 376 
Marriote, blind spot of, 337 
Media, refracting, 43; opacities 

in, 69 
Mod inn plane. 208 
Medullated nerve-fibres, 76 
Menopan-o complicated by pres- 
byopia, 364 
Metabolism affected by eye- 
strain, 364 
Mel re angle, 224 
.Metric numbering of lenses, :!<> 
.Migraine from eyestrain, 3tfl 
Miot i<--. hi."), See Eat rim . 
Mire- of ophthalmometer, 177 
M in or-. ;i\i- of, 7 

concave, 7 

com 

foci 

images in. <;. 6 

retinoscopic, 90 
Mixed astigmatism, 170, 191 



388 



INDEX. 



Motility of eyes, 209, 219. See 
also Muscles, Nerve-cen- 
tres. 
conjugate movements, 235 ; 
measurement of, 235,239. 
See Conjugate. 
fusion powers, 226; tests of, 

226, 233. See Fusion. 
position of rest, 219; tests of, 
219, 226. See Position 
of rest. 
Motor oculi nerve, 214 
Movement of redress, 217 
Muscae volitantes in myopia, 

150, 153 
Muscles ocular, action of, 210 
advancement of, 289 
anatomy of, 210 
extrinsic, 210 
intrinsic, 301 
insertions of, 211 
insufficiency of. See Heter- 

ophoria, 242 
motion limited by tenot- 
omy, 291 
motion after paralysis, 297 
paralyses of, 206 
tenotomy of, 289, 291 
Mydriastis, 115 
paralytic, 301 
Mydriatics, 115. See Cyclople- 

gics. 
Myopia, 62, 144 

an occupational disease, 146 
apparent in spasm of accom- 
modation, 145, 154 
apparent, simulated by hy- 
peropia, 140 
axial, 145, 156 

.use of, 145, 146 
causing exophoria, 147, 280 
choroid in, 148, 149 
ciliary muscle in, 149 
complications of, 150, 152 
corneal radius in, 155 
correction , 156, 159, 162 
crescent in, 149 
detachment of retina in, 150, 
153 



Myopia, determination of, 157, 

158 
developing in senile cataract, 

144 
elongation of eye in, 145 
estimation of, 157 
far point in, 151 
glasses in, 160 
haemorrhage in, 150, 153 
hereditary, 145 
hygiene of, 155 
in schools, 146 
iodides in, 161 
lenticular, 145 
long radius in, progressive, 

155 
morbid anatomy of, 148 
muscae in, 150, 153 
near point in, 151 
objective measurement of, 

157 
physiological excavation in, 

149 
postponing presbyopia, 152, 

196 
predisposition to, 146 
prevalence in professions and 

races, 146 
prognosis, 154 
progressive, 145, 154 
prophylaxis of, 155, 160 
racial tendency to, 146 
reduction by removal of lens, 

161, 204 
refractive, 144 
region in, 151 
retinoscopy in, 94, 95, 157 
rules for prescribing in, 155 
scleritis in, 148 
staphyloma postlcum in, 148, 

150 
stationary, 154 
symptoms of, 150, 153 
theory of, 146 
treatment of, 155, 159, 162; 

operative, 161 
vision in, 150, 153 
Myopic astigmatism. See As- 

-tigmatism, myopic. 



INDEX. 



389 



Near point. ,32. 60, 195 

determination of. 195 

in emmet ropia, 151 

in hyperopia, 151 

in myopia, 151 

recession of in presbyopia, 

195 
vision tests, 60, 61 
Nerve head, 73 

Nerves, cortical centres of, 215 
fusion of centres of, 215 
nuclear centres of, 214 
of muscles, 214 
optic, 73, 347. See Optic 

nerve. 
volitional centres of, 215 
Neurasthenia from eye-strain, 
362 
field in, 348 
Neutral band. 326 
Neutralizing lenses, 31 
Nodal point of eye, 45 

of lens, 19 
Nuclear paralysis. 303 
Numbering lenses, 29 

Objective tests, disadvantage of, 

123 
Oblique inferior, 213 
action of, 213 
paralysis of, 307 
spasm of. 293 
Oblique superior, 212 
action of, 212 
paralysis of, 307 
Old paralyses, 300 
Opacities in media, 69 
Localization, 71 
of cornea, 69, 70 
of lens, 70 
of vitreous, <»7 

recognition of, by ophthalmo- 
scope, 69 
< tpaque uerve-fibres, 76 
Operations, advancement, 253, 
289 
for cyclophoria, 209 
partial tenotomy, 249. 269, 
290 



Operations, shortening, 252 

tenotomy, 250, 291 

uncertainty of, 292 
Ophthalmometer of Javal, 174 

advantages of and disadvan- 
tages of, 175, 181 

in astigmatism, 180 

radius of cornea by, 178 
Ophthalmoplegia, externa, 300 
Ophthalmoplegia, interna, 301 

totalis, 300 
Ophthalmoscope, 62, 65 

electric, 68 

Loring, 66 
Ophthalmoscopy, 62 

advantages of indirect method, 
86 

chorioidal ring, 74 
vessels, 76 

differences of level in, 84, 87 

direct method of, 69 

estimation of refraction bv, 
78, 80, 83 

fundus seen by, 70 
reflex in, 76 

image in, 72 

in astigmatism, 83 

in hyperopia, 79 

in myopia, 80 

indirect method of, 84 

lamina cribrosa, 73 

medullated fibres, 76 

nerve head, 73 

opacities in media, 70 

pulsation in vessels, 75 

pupillary reaction in, 69 

retinal vessels, 75 

scleral ring, 74 
Optic axis, 44, 208 

area, 350 

centre of lens. 19, 31 

disk, 39 

foramen. 350 

groove, -'i")') 

thalami, 350 

tracts, :;is 
( ijit if nerve, 7:: 

anatomy, •') 19 

atrophy of field in. '■', \A 



390 



INDEX. 



Optic nerve, chiasm of, 350 

excavation in atrophy, 74 
in glaucoma, 74 
physiological, 74 

oval in astigmatism, 174 

vessels of, 75 
Optical centre, 19, 31 

system of eye, 43 
Optometer hair, 61 
Ora serrata, 37 
Orientation, 334 

false in paralysis, 297 
Orthophoria, 242 

Papilla. See Nervve head. 
Parallactic displacement of 
opacities, 70 
of fundus, 87 
Parallax test, Duane's, 222 
Paralysis, 297 

abnormal position of head in, 
299 

basal, 302 

causation of, 301, 303 

congenital, 301 

con j ungate, 316 

contraction in old, 300 

cortical, 316 

deviations in, 297 

diabetic, 301 

diagnosis of, 303, 308, 310 

diplopia in, 298 

false image, 304 

false projection in, 298 

fascicular, 302 

from injury, 301 

intracranial 302 

limitation of motion in, 297 

localization, 303, 316, 317 

measurement of, 311 

nuclear, 303 

oculo motor, 297 

of abducens, 308 

of accommodation, 53, 300 

of associated movements, 315 

of convergence, 315 

of divergence, 316 

of externus, 307 

of extrinsic muscles, 300 



Paralysis of inferior oblique, 
307 

of inferior rectus, 309 

of internus, 308 

of oblique muscles, 307 

of pupil, 53, 301 

of superior oblique, 306 

ol superior rectus, 309 

of trochlear nerve, 307 

old, 300 

operations for, 313, 314 

orbital, 302 

post-diphtheritic, 301 

primary deviation in, 277 

prisms in treatment of, 313 

rheumatic, 301 

sclerotic, 301 

secondary deviation in, 297 

svpbilitic, 301, 312 

tabetic, 301, 312 

toxic, 301, 312 

treatment of, 312, 317 

vertigo in, 299 
Partial tenotomy, 290 
Perception of color diminished 
in nerve disease, 343 

theories of, 302, 303 
Perception of light, how tested, 

335 
Perimeter, 335 

charts, 338 

in measuring field, 335 

in measuring paralysis, 312 

in measuring rotation of eyes, 
236 

in measuring strabismus, 273, 
275 
Periodic strabismus, 276 
Peripheral scotomata, 341 

vision, 334, 335 
Periscopic lenses, 36 
Phorometer, 220 
Pigments, color, 320 
Pilocarpine, 125. See Miotics. 
Plaoidos disk, 175 
Points, cardinal, 45 

corresponding, 207 

far, 52, 151 

near,- 52, 151, 195 



INDEX. 



391 



Points of reversal, 96 

principal, 45 
Position of rest, 208, 219 

cover test. 225 

Duane's test, 222 

equilibrium test, 226 

Hardy test, 222 

in accommodation, 224 

Maddox double prism, 224 

Maddox rod, 220 

metre angle, 224 

of vertical planes, 223 

prism tests, 219 

screen tests, 225 

Stevens' lens, 222 
Prentice prism nomenclature, 

15 
Presbyopia, 194 

bifocal lenses in, 199 

compensation for, 195 

complicated by astigmatism, 
197 
by hyperopia, 196 
by myopia, 196 

near point in, 195 

region in, 198 

symptoms of, 195 

treatment of, in ametropia, 
198 
of in emmetropia, 197 
Primary colors, 321 

deviation in paralysis, 297 
in squint. 270 

position of eyes, 208. See 
Position of rest. 
Principal axis, 7, 19 

focal distance, 7, 45 

focus. 19 

points. 45 
Prism convergence, 227 

dioptre, 15 

divergence, 230 
Prism tests, — 

Duane's, 222 

equilibrium, 219 

Graefe, 226 

Maddox, 224 

of binocular vision, 217 

of convergence, 227 



Prism tests: of deorsumverg- 
ence, 231 
of divergence, 230 
parallax, 222 
Prismatic effect of lenses, 162, 

203, 248, 260 
Prisms, 13 
action of, 13 
for constant use, 249, 266, 

268 
for exercise, 247 
for hyperphoria, 266 
for paralysis, 312 
in heterophoria, 219, 222, 227, 

247, 248 
in paralysis, 311, 313 
neutralizing, 16 
numbering, 15 
overcoming of, 227 
rotary, 229 
Progressive myopia, 145. See 
Myopia. 
diagnosis, 155 
long corneal radius in, 155 
Projection and orientation, 305, 
334 
of field of vision, 334 
Pulsation of retinal vessels, 75 
Punctum proximum. See Near 
point. 
remotum. See Far point. 
Pupil, 42, 111 
anisocoria, 113 
Argyll -Robertson, 115 
consensual reaction of, (54, 

154 
contraction of, through third 
nerve, 111 
in corneal perforation, 112 
dilation of, caused by sympa 
thetic, 112 
by emotion, 112 
by glaucoma, 112 
distortion of by synechia, <'>~>. 

69 
effed of drugs on. ] 12 
examinal ion of w it h ophthal- 
moscope, 69 



392 



INDEX. 



Pupil, inequality of, always 
pathological, 113 
luminosity of, 65, 69 
miosis, 115, 126 
reaction of, with convergence 
and accommodation, 111 
as evidence of light per- 
ception, 114, 369 
consensual, 64, 114 
in tabes, 115 
to light, 60, 64, 114 
to poisons, 112 
to sensory stimuli, 112 
size of, 114 
Purkinje-Sanson reflexes, 64 
Purple, visual, 40 

Radius of cornea, 154, 174 

in progressive myopia, 155 

measurement of, 155, 178 

of reduced eye, 45 
Range of accommodation, 52 
Rectus externus, 210 

inferior, 212 

internus, 210 

superior, 211 
Red, waning perception of, in 
nerve disease, 346 

blindness, 325, 326, 328, 331 
Red-green visual substance, 323 
Redress, movement of, 217 
Reduced eye, 45 
Reflection, 3 

from concave mirror, 7 

from convex mirror, 9 

from plane mirror, 5 

irregular, 6 

laws of, 3 

total, 14 
Reflex, corneal, 65 

fundus, 65, 69, 71, 76 

lenticular, 65 

macular, 77 

pupillary, 69 
Purkinje-Sanson, 65 

retinal, 67, 88, 92 

scissors, 107 

symptoms in eye-strain, 245, 
355, 365 



Refracting angle, 13 

media, 43 
Refraction, 9 

estimated by ophthalmom- 
eter, 174 
by ophthalmoscope, 78, 80, 

83 
by retinoscopy, 88. See 

Retinoscopy. 
by trial case, 122, 137, 143, 
159, 182, 193, 198 

index of, 11, 43 

laws of, 9, 10, 11 

static, 122 
Refractive myopia, 145 
Region of accommodation, 52 

in hyperopia, 151 

in myopia, 151 
Regular astigmatism, 165 
Relative scotoma, 341 
Removal of lens in myopia, 161, 

204 
Resection of muscle, 251 
Retina, 38 

appearance of with ophthal- 
moscope, 74 
. arteries of, 75 

medullated nerve fibres in, 76 

veins -of, 75 
Retinal images, 46. See Vision. 
Retinoscopy, 88 

actual motion of reflex in, 91 

aerial image in, 94 

apparent motion of reflex, 91 

arrangement of light in, 91 

difficulties of, 104 

in astigmatism, 98, 101, 173 

in emmetropia, 92, 96 

in high myopia, 94 

in hyperopia, 93, 97, 134 

in low myopia, 95 

in myopia, 157 

plane mirror in, 90 

point of reversal in, 96 

reflex in, 88 

schematic eye in, 104, 105 

scissors reflex in, 107 

spherical aberration in, 104 



INDEX. 



393 



Betinoscopv without cvcloplesria, 
107,* 135, 138/ 140, 157, 
182 
Ring, ehorioidal. 74 

scleral. 74 
Rotations of eyeball, 236 
Rotations, measurement, 236 
by perimeter, 236 
by tropometer, 238 

Schematic eye. 104 
Scintillating scotomata in mi- 
graine, 361 
Scissors reflex, 107 
Sclera, anatomy of, 37 
Scleral ring, 74 
Sclero-chorioiditis in myopia, 

147 
Scoliosis from eye-strain, 364 
Scopolamin, 115. See Cyclo- 

plegics. 
Scotoma. 343 

absolute, 343 

central, 343 

color. 345 

in chorioiditis, 347 

in detachment of retina, 347 

in glaucoma, 343 

in retrobulbar neuritis, 347 

negative, 345 

peripheral, 343 

positive, 345 

relative, 345 

Bcintillans, 361 
Screen test, 217, 222. 225 
Secondary axes, 7, 19 

deviation in strabismus, 270 
in paralvsis. 297 

foci, 8, H)" 
Sensations, color, 321 
Sensitiveness of cornea) c>\ 
Shadow tost, 88. See Retin 

oscopy. 
Shortening of rectus, 251 
Sixth nerve, 214 

paralysis of. 307 
Skiascopy, 88. See Retinas- 

copy. 
Snellen test types, 57, 60 



Spa sin of accommodation, 140 

simulating myopia, 145, 154 
Spectroscope in color blindness, 

319, 325 
Spectrum, 319 
Specular reflection, 6 
Spherical aberration, 42, 104 
Sphincter of iris, 42 
Squint. See Strabismus, 270 
Staphyloma in myopia, 148. 150 
Static refraction, 121 
Stereoscopic vision, 216. See 
Binocular vision. 

perimetry, 341 
Stereoscope, 343 

in malingering, 373 
Stevens clinoscope, 232 

phorometer, 221 

tropometer, 238 
Strabismus or squint, 270 

accommodative, 279 

alternating, 275 

amblyopia in, 274, 281, 293 

apparent, 275 

binocular, 279, 287 

concomitant, 270 

congenital, 276 

convergent, 275. 287 

deviation, primary and sec- 
ondary. 270 

divergent, 275, 279 

etiology of, 276 

fusion in, 277, 279, 281 

hyperopia and, 278 

latent, 242 

measurement of, 271, 287 

monocular. 288 

myopia, influence of, 280 

oblique, 293 

paralytic, 296 

periodic, 276 

refraction a factor in, 278 

suppression of image In, 273 

spontaneous cure of, 282 

teste of cure, 295 

1 reatmenl of. 282 \ refract ion, 
fusion, 284; opera- 
tive, 289; when ambly- 
opic, 293 



^^/vv^^wa/ 



394 



I 

INDEX. 



2 1 



Subjective refraction in astig- 
matism, 182 to 193 
in hyperopia, 137, 143 
in myopia, 159 
in presbyopia, 198 
Subjective refraction, static, 

122, 205 
Subnormal accommodation, 196 

Tabes, Argyll-Robertson pupil 
in, 115 

causing paralysis of ocular 
muscles, 301 
Tangent curtain, 338 
Temporal hemianopia, 351 
Tenotomy, 289, 291 

graduated, 249, 250, 290, 291 

over effect of, 291 
Test types, 48, 57 

Jaeger, 60 

Snellen, 57, 60 
Testing functional, 58 
Third nerve, 214 

paralysis of, 297 
Tipping of images in oblique 

paralysis,, 306 
Tobacco amblyopia, 345 
Toric lenses, 36 
Torsion, 209, 212, 231 

clinoscope in, 232 

Maddox rod, 23*1 
Toxic amblyopia, 347 

paralysis, 301 
Trial case, 61. See Subjective 
refraction. 

frame, 29, 61, 169 
Trichromic vision, 324 
Trochlear nerve, 215 
paralysis of, 307 



Tropometer, Stevens, 328 
in heterophoria, 238 
in paralysis, 310 
in strabismus, 257, 2Qo, 267 

Valk twin hook, 253 
Vertical diplopia, 305 

squint, 275, 292 
Vertigo in paralysis, 299 
Virtual focus, 23 

image, 22, 23 
Vision, binocular, 206 

central, 206 

in astigmatism, 171 

in hyperopia, 128 

in myopia, 150, 153 

peripheral, 335 
Visual acuity, 47 

angle, 46 

line, 208 

minimum angle, 47 

purple, 40 
Vitreous humor, 41 
opacities in, 69, 70 

Wernicke's hemianopic pupil- 
lary, 351 

White-black visual substance, 
326 

Williams' color lantern, 329 

Worsted test for color sense, 
329 

Worth amblyoscope, 284 

Yellow spot, 76 

Young-Helmholz theory of color 
sense, 322 

Zone of Zinn, 41 



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